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Article

Performance Evaluation of Readily Available Iron–Carbon Micro-Electrolysis Materials for Domestic Sewage Treatment

by
Yong Xu
1,2,†,
Xiaojiao Ren
3,4,†,
Di Wu
3,4,
Xuejin Zhou
3,4,
Yanping Liu
1,2,*,
Shanshan Sun
1,2,5,
Jimeng Feng
1,2,5,
Jian Shen
1,2,5,* and
Xinze Wang
1,2,5
1
Shanghai Jiao Tong University Yunnan Dali Research Institute, Dali 671000, China
2
National Observation and Research Station of Erhai Lake Ecosystem in Yunnan, School of Environmental Science and Engineering, Shanghai Jiao Tong University, Dali 671000, China
3
Yunnan Communications Investment Eco-Tech Co., Ltd., Kunming 650000, China
4
Yunnan Provincial Communications & Investment Construction Group Co., Ltd., Kunming 650000, China
5
School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Appl. Sci. 2025, 15(23), 12355; https://doi.org/10.3390/app152312355
Submission received: 26 October 2025 / Revised: 18 November 2025 / Accepted: 20 November 2025 / Published: 21 November 2025
(This article belongs to the Special Issue Advances in Pollutant Removal from Water Environments)
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Abstract

This study systematically evaluated the removal performance of iron–carbon (Fe/C) micro-electrolysis materials with different proportions and preparation methods for nitrogen (N), phosphorus (P) and chemical oxygen demand (COD) in domestic sewage. This study investigated the effects of different Fe/C ratios, hydraulic retention time (HRT), raw materials and sintering bonding conditions on the efficiency of domestic sewage treatment through both static and dynamic experiments. In the static experiments, iron filings (IFs), steel slag (SS), and coconut shell carbon (CSC) were physically mixed, whereas the dynamic tests simulated the continuous treatment of domestic sewage. The results indicated that the Fe/C materials effectively removed P, particularly materials with high Fe/C ratios, which achieved removal rates of 96–98%. The COD removal efficiency of low Fe/C ratio material was better, reaching a removal rate of more than 70% under the optimal conditions. For Fe/C physical mixed materials, SS replacing IFs had excellent performance in ammonia nitrogen (NH4+-N) removal (>93%), but other indicators were poor, which limited its application. Results from the continuous flow experiment indicated that the physically mixed filler with an Fe/C mass ratio of 2:1 showed excellent and stable TP and COD removal rates (98.6% and 92.8%) for the actual domestic sewage. In addition, the Fe/C micro-electrolysis filler sintered at 400 °C using kaolin as a binder exhibited good potential for pollutant removal, providing a feasible solution for reducing energy consumption. This study provides important data support for the development of low-cost and efficient decentralized rural sewage treatment technology.

1. Introduction

In rural areas, sewage treatment primarily relies on technologies like Membrane Bioreactor (MBR) and the Anaerobic/Anoxic/Oxic (A2/O) process. However, these systems require skilled operators, limiting their environmental advantages. On the other hand, properly planned constructed wetlands offer a sustainable solution [1]. The Fe/C micro-electrolysis materials possess multiple functions including adsorption, coprecipitation, and reactive oxygen species generation, granting them distinct advantages over traditional functional water treatment media and making them a growing research focus in wastewater treatment. The potential of Fe/C materials is particularly promising for tackling rural sewage challenges, offering practical solutions for cleaner water. Typically, iron with a negative redox potential serves as the anode, while carbon acts as the cathode. The inherent 1.2 V potential difference enables the electrolytic degradation and removal of pollutants [2], accompanied by a series of physical (electrostatic adsorption, hydrophilic effects, π-π interactions, hydrogen bonding) and chemical reactions (reduction reactions, ion exchange, covalent bonding, direct reactions with surface functional groups) [3,4].
Ren [5] achieved a total phosphorus (TP) removal efficiency of 96% to an ideal 100% by packaging surface soil, grain-type polymer-poly-butylene, iron powder (Fe), and biochar powder (C) in each soil mixture block jute bag at a dry-weight ratio of 7:1:1:1. In a continuous-flow constructed wetland experiment incorporating Fe/C materials [6], the average removal rates of NH4+-N, nitrate nitrogen(NO3−-N), total nitrogen (TN), and phosphate (PO43−) were 45.68%, 92.16%, 85.76%, and 87.00%, respectively. The Zeolite Fe/C constructed wetland designed by Li demonstrated superior NH4+-N removal efficiency (77.66~92.23%) while effectively inhibiting NO3−-N accumulation [7]. Hu [8] enhanced an anaerobic-aerobic-anoxic baffled reactor with Fe/C micro-electrolysis materials, achieving removal rates of 71.9%, 60.5%, 93.3%, and 85.2% for TN, NH4+-N, TP, and COD, respectively, under optimal operating conditions (DO = 1.98 ± 0.14 mg/L, HRT = 24 h, influent C/N = 7.0 ± 0.8). A mixture of red mud and pine wood (mass ratio 1:1, 800 °C, pH = 4.5) exhibited a maximum ibuprofen adsorption capacity of 21.01 mg/g [9]. Functional composites formed by co-pyrolysis of red mud and corn straw (mass ratio 1:1, 700 °C, pH = 1) demonstrated excellent performance in treating acidic dye wastewater, with an adsorption capacity of 70.9 mg/g [10]. Fe/C composites synthesized via co-pyrolysis of red mud and pomelo peel (mass ratio 1:1, 800 °C) achieved a removal capacity of 32 mg/g for anionic azo dyes in water [11]. When the Fe/C mass ratio was 1:5, the methyl orange removal efficiency reached 98.5% [12].
Biochar serves as a porous carrier for iron, where a mixture of carbon powder and iron powder is blended in a specific ratio using a binder and then shaped through firing. The structured filler enhances contact between the anode and cathode, exhibiting excellent electron transfer capabilities while effectively preventing separation and compaction issues. Sintered fillers have evolved into the primary commercial filler option. High-temperature sintering, typically conducted at temperatures above 800 °C [9], is a commonly employed pyrolysis method, though its high energy consumption remains a drawback. Research on low-temperature sintered (≤400 °C) Fe/C materials for wastewater treatment is currently limited. However, in rural areas, where economic development lags and government funding for infrastructure construction is severely inadequate, the high costs of wastewater treatment facilities are often prohibitive. There is an urgent need to explore cost-effective, low-maintenance, efficient, and stable technological solutions. Traditional soil infiltration systems for wastewater treatment, while low in construction costs, exhibit poor resistance to shock loads. Conventional arrangements of soil mixed blocks suffer from issues such as limited flow distribution and mass transfer efficiency, low effective volume ratios, stagnant zones, and short-circuiting, leading to suboptimal nutrient removal efficiencies. Additionally, spatial imbalances between aerobic nitrification and anoxic zones in the infiltration layer, as well as clogging problems, result in insufficient removal of NH4+-N and TN [13,14,15]. Moreover, many fillers are either scarce or expensive [16], hindering their widespread adoption in rural areas. There is a pressing need for an alternative, cost-effective filler. Fe/C micro-electrolysis materials offer a potential low-cost solution for rural wastewater treatment. Without the need for sintering, both iron scraps and biochar raw materials are inexpensive, making them easily applicable and promotable in rural regions.
This study explored two approaches for rural domestic wastewater treatment: one involved the direct use of physically mixed Fe/C materials, while the other employed materials processed through different preparation methods, such as high-temperature (800 °C) and low-temperature (400 °C) sintering. The research investigated the effects of varying iron sources, Fe/C ratios, and treatment durations on the removal efficiencies of COD, NH4+-N, TP, and NO3−-N. It aims to evaluate the pollutant removal performance of Fe/C micro-electrolysis materials, identify optimal process parameters, and develop a cost-effective rural wastewater treatment solution based on the Fe/C micro-electrolysis method.

2. Materials and Methods

2.1. Fe/C Micro-Electrolysis Material Experimental Setup

The setup consisted of a plexiglass column measuring 12 cm × 50 cm (diameter × height). The filling material was a uniformly mixed Fe/C mixture (IFs and coconut shell charcoal) with a mass ratio of 2:1. A 0.5 mm iron mesh was placed every 5 cm vertically to prevent vertical separation or compaction of the IFs and biochar particles. The particle size of the Fe/C mixture ranges approximately from 0.5 to 0.8 cm, with a packing volume of 2 L per reactor. In the continuous vertical flow experiment, the peristaltic pump tubing has an inner diameter of 4 mm, a rotor diameter of 40 mm, and operates at a speed of 4.4 rpm, processing approximately 10 L/d of water sample.

2.2. Experimental Materials

The IFs were purchased from Shijiazhuang Yifeng Mineral Products Trading Co., Ltd., Shijiazhuang, China, sourced from workshop cast IFs. The carbon powder was purchased from Changge Carbonor Catalytic Technology Co., Ltd., Xuchang, China, produced by high-temperature pyrolysis of coconut shells. The SS samples were of two types: provided samples (diameters of 12 mm and 7 mm) and commercially purchased samples (diameter of 3 mm). The experimental wastewater consisted of simulated wastewater and influent from the Chengjiang Wastewater Treatment Plant in Yuxi City, China.
Reagents:
Potassium dihydrogen phosphate (KH2PO4), analytical grade, is used to prepare phosphate for simulated wastewater. Potassium hydrogen phthalate (KHC8H4O4), analytical grade, used to prepare COD for simulated wastewater. Ammonium chloride (NH4Cl), high purity grade, used to prepare NH4+-N for simulated wastewater. Potassium nitrate (KNO3), analytical grade, used for NO3−-N for simulation.

2.3. Experimental Design

2.3.1. Static Removal Experiment of IFs and CSC with Different Ratios

To investigate the pollutant (N, P, COD) removal ability of Fe/C materials with varying ratios at different residence times, a pre-configured simulated urban sewage treatment plant effluent was used. The effluent contained PO43−, COD, NH4+-N, and NO3−-N at concentrations of 0.481 mg/L, 54.4 mg/L, 9.91 mg/L, and 3.27 mg/L, respectively. Follow IFs and CSC according to low iron-to-carbon ratio (L-Fe/C, Fe/C = 1:3, 1:2)), medium iron-to-carbon ratio (M-Fe/C, Fe/C = 1:1), and high iron-to-carbon ratio (H-Fe/C, Fe/C = 2:1, 3:1). After physically mixing the mass ratios of 1:3 (L-Fe/C), 1:2 (L-Fe/C), 1:1 (M-Fe/C), 2:1 (H-Fe/C), and 3:1 (H-Fe/C), place the Fe/C material in a beaker, add the same mass of known concentration solution, and let it stand for 24 h, 48 h, and 96, respectively.

2.3.2. Effect of SS as a Substitute for IF

To examine the pollutant removal capacity of low-cost SS when physically mixed with CSC, and when SS is used as a substitute for IFs, the particle sizes (P.S.) of SS were set at 3 mm, 7 mm, and 12 mm, respectively. The experimental procedure followed the method described in Section 2.3.1. Additional L-Fe/C groups (2:3 and 3:4) were included. Release experiments were conducted for CSC, SS, and IFs, with a raw material-to-pure water mass ratio of 1:1. The release concentrations were measured after standing for 24 h, 48 h, and 96 h.

2.3.3. Continuous Domestic Wastewater Treatment Experiment

Static adsorption experiment for high-concentration wastewater. The ratios of IF to CSC were set at 2:1, 2:3, and 3:2, with an additional group of 1:1:1 (IF: CSC: SS). The static adsorption durations were 24 h, 48 h, 96 h, 168 h, and 240 h.
Dynamic treatment experiment. The test water was collected from the influent of the Chengjiang Domestic Wastewater Treatment Plant. A vertical flow setup (Figure 1) was employed for the dynamic adsorption study, with a controlled HRT of 4.8 h.

2.3.4. Performance Differences in Fillers Prepared at Various Temperatures and Ratios

To investigate the treatment efficiency of domestic wastewater by Fe/C materials prepared at different temperatures and ratios, the optimal ratio of Fe/C material was mixed with specific proportions of kaolin, bentonite, ammonium bicarbonate, quartz sand, lime, and aluminum powder (Table 1). Among them, Type IV consisted of IFs and CSC, while the other types used high-purity iron powder and reed carbon. Water, equivalent to approximately 25% of the total mass of the Fe/C microspheres, was added and stirred until a plastic paste formed, at which point water addition was stopped. Stirring continued until complete homogenization was achieved. Spheres with a diameter of 2 cm were prepared and air-dried for 24 h. The dried Fe/C microspheres were then placed in a sintering furnace (Model: Energy-efficient box furnace, SX-G12163), where they were heated to 400 °C and 800 °C at a rate of 5 °C/min under an oxygen-free environment and held at these temperatures for 1 h. After the holding period, the microspheres were allowed to cool naturally to room temperature, completing the preparation of the Fe/C microspheres. The prepared Fe/C micro-electrolysis materials were placed into 500 mL of synthetic domestic wastewater for treatment experiments. The treatment durations were set at 24 h, 48 h, 72 h, 96 h, 120 h, 168 h, and 216 h.

2.4. Experimental Methods and Analysis

The removal rates of pollutants on the Fe/C materials were calculated according to Equation (1).
η   =   C 0   −   C a C 0 ⋅ 100 %
where η is the removal rates of pollutants (e.g., COD, PO43−, NH4+-N and NO3−-N) by Fe/C materials. C0 is the initial pollutant concentration of the solution, mg/L. Ca is the pollutant concentration of the solution at a specific contact time (e.g., 24 h, 48 h, 72 h), mg/L.
The desorption capacity [17,18,19,20] of raw materials was calculated according to Equation (2).
Q   =   ( C t   −   C 0 ) V m
where Q is the pollutants desorption capacity of raw materials (e.g., IFs, SS and CSC), mg/g. C0 is the initial pollutant concentration of the solution, mg/L. Ct is the pollutant concentration of the solution at a specific contact time (e.g., 24 h, 48 h, 96 h), mg/L. V is the volume of the solution, L. And m is the mass of raw materials, g.
During the experimental process, the COD was measured with a spectrophotometer (DR3900, HACH Company, Loveland, CO, USA) using potassium dichromate as oxidant (HJ/T 399-2007 [21], Chinese national standard). The content of TP and PO43− was determined at 700 nm by ammonium molybdate spectrophotometry (GB 11893-89 [22], Chinese national standard), the content of TN was determined at 220–275 nm by alkaline potassium persulfate digestion UV spectrophotometry (HJ/T 636-2012 [23], Chinese national standard), the content of NH4+-N was determined at 420 nm by Nessler reagent method (HJ/T 535-2009 [24], Chinese national standard), and the content of NO3−-N was determined at 220-275 nm by UV spectrophotometry (HJ/T 346-2007 [25], Chinese national standard). Use Excel for data statistics and Origin Pro 2020 for plotting. For sample testing, we adhered rigorously to China’s “Technical Specifications Requirements for Monitoring of Surface Water and Waste Water” (HJ/T 91-2002 [26]), which requires that “10% of samples in each batch must be tested in parallel duplicates.” Moreover, every batch of experimental data was validated using certified reference materials to ensure accuracy.

3. Results

3.1. Static Simulation Experiment for Phosphorus Removal via Fe/C Physical Mixture

3.1.1. Phosphorus Removal Performance

The iron-to-carbon ratio (Fe/C) is a critical factor influencing phosphorus removal efficiency. Higher iron content enhances phosphate removal capacity, as phosphorus can be immobilized or precipitated by iron oxides and oxhydroxides generated from iron-containing materials, whereas the negative surface charge of biochar is unfavorable for adsorbing similarly charged phosphates [27,28]. At an adsorption time of 24 h, the phosphate removal efficiency exceeded 80% for all tested ratios (Figure 2a). The H-Fe/C material exhibited the highest phosphate removal performance, achieving a removal rate of 96–98%. Even at a Fe/C ratio of 1:1, the removal rate remained as high as 95%. The L-Fe/C materials (1:2, 1:3) showed a phosphate removal efficiency of approximately 82%. The optimal adsorption performance was observed at 24 h. At 48 and 96 h, the adsorption efficiency declined, and the Fe/C materials began releasing previously adsorbed phosphates. Residence time significantly affects the phosphate adsorption capacity of Fe/C materials. Previous studies [29,30] indicate that phosphate adsorption by Fe/C materials primarily occurs during the initial stage. Negatively charged phosphates are rapidly adsorbed via electrostatic attraction to positively charged iron ions (Fe2+/Fe3+), resulting in high phosphate removal efficiency (>90%) within 24 h. However, prolonged contact time leads to excessive OH− accumulation on the Fe/C materials surface, shifting the surface charge from positive to negative. Consequently, electrostatic repulsion occurs between the negatively charged material surface and phosphate anions, significantly reducing adsorption capacity.

3.1.2. Organic Matter Removal Performance

As shown in Figure 2b, in the static simulation experiment, L-Fe/C ratio demonstrated superior COD removal efficiency. The optimal performance was achieved at a Fe/C ratio of 1:2 with a retention time of 96 h, reducing the solution concentration from 54.4 mg/L to 13.4 mg/L. In contrast, H-Fe/C materials exhibited poor COD removal. Particularly at an Fe/C ratio of 3:1, the desorption effect of the material outweighed adsorption, leading to an increase in COD concentration to 67.2 mg/L. COD degradation primarily occurs through galvanic coupling, redox reactions, coagulation, and precipitation. Fe3+ enhances COD removal by interacting with organic substances such as carboxyl groups [31]. Additionally, ferrous iron and ferric hydroxide formed via iron oxidation and precipitation can adsorb and precipitate organic pollutants. The observed release of COD at this ratio may originate from residual organic matter in the iron material itself or the generation of reaction byproducts. Table 2 shows that the Fe/C materials likely include residual carbon, grease, or surface impurities that were not fully oxidized. This explains why Some effluent COD concentrations are higher than the initial concentration at an Fe/C ratio of 3:1.

3.1.3. Nitrogen Removal Performance

As shown in Figure 2c, the Fe/C material exhibited good adsorption efficiency for ammonia nitrogen, with longer retention times resulting in higher adsorption capacities. At a retention time of 96 h, the removal rates of ammonia nitrogen by L-Fe/C and M-Fe/C materials ranged from 83% to 87%, while H-Fe/C showed a relatively lower removal rate (72~73%). When the Fe/C ratio was 3:1, prolonged time instead promoted desorption. This may be attributed to the hierarchical pore structure of biochar providing abundant adsorption sites, but as these sites gradually became saturated, the performance declined. This trend indicates minimal biofilm formation, suggesting that physical adsorption is the primary removal mechanism for NH4+-N at this stage [32]. The adsorption of nitrate nitrogen was less effective due to the lack of microbial activity and insufficient carbon sources [33], which inhibited denitrification. Overall, the optimal removal efficiency for nitrogen, phosphorus, and organic matter was achieved at a Fe/C ratio of 3:1 and an HRT of 24 h.

3.2. Static Removal Simulation Experiment of SS as a Substitute for IF

3.2.1. Treatment Effects on N, P, and COD

After replacing IFs with SS, the P.S., treatment time, and Fe/C ratio collectively determine the adsorption efficiency of the Fe/C material for N, P, and COD (Figure 3). The P.S. has the most significant influence. When P.S. = 3 mm, the Fe/C ratio and soaking time play a certain role, but the desorption effect outweighs the adsorption effect. The L-F/C group consistently exhibits desorption behavior, with lower Fe/C ratios, smaller P.S., and longer soaking times resulting in greater desorption amounts. Under an initial concentration of 0.58 mg/L, the phosphate concentration reaches 12.4 mg/L when P.S. = 3 mm, Fe/C = 1:3, and HRT = 96 h, with a concentration difference of 11.82 mg/L released by the material itself. As P.S. increases, the M-F/C and H-F/C groups show gradually enhanced adsorption effects under prolonged soaking times. At P.S. = 7 mm, Fe/C = 3:1, and HRT = 96 h, the phosphate removal efficiency reaches 88%. The adsorption performance of other groups for phosphate is inferior to that of IFs.
The treatment effect on COD was slightly better than that on phosphate, with the L-F/C group showing relatively superior adsorption performance. However, in the M-Fe/C and H-Fe/C groups, desorption outweighed adsorption. This may be attributed to the dominant role of electrolysis and flocculation precipitation within the IFs when their dosage was high. As the Fe/C ratio decreased, the intrinsic effect of the IFs weakened, while the galvanic cell interaction with biochar became more pronounced. When the Fe/C ratio was <1:1, Fe/C micro-electrolysis played a dominant role, progressively enhancing the capacity for organic matter removal [34]. At P.S. = 12 mm, HRT = 96 h, and Fe/C = 1:3, the COD removal rate reached 70%. Smaller P.S. and longer adsorption durations correlated with stronger COD adsorption capacity (Figure 3b).
As shown in Figure 2c, the Fe/C material exhibited effective adsorption of NH4+-N, with particle size exerting minimal influence. Prolonged retention time increased adsorption capacity, achieving a NH4+-N removal rate exceeding 93% at 96 h—outperforming Fe/C materials composed solely of IFs. Nitrate nitrogen, however, displayed different behavior (Figure 3d). At 48 h, the nitrate nitrogen removal rate ranged from 5% to 7%, showing little difference from materials using IFs as the raw component. Yet, as time progressed, desorption intensified, ultimately leading to net release. Throughout the experimental phase, NH4+-N removal primarily relied on physical adsorption.

3.2.2. Release of N, P, and COD from Materials

After mixing the raw materials with pure water and letting them stand for several hours, the release results are shown in Table 2.
CSC inherently contains a significant amount of phosphorus, and its release rate keeps climbing significantly as time passes, jumping from 0.016 mg/g to 0.021 mg/g. The primary reason for “reduced adsorption effectiveness due to high carbon content” lies in the fact that carbon materials themselves are significant sources of phosphorus release. The release concentrations of COD and NH4+-N increased during the 24 h to 48 h period, and the COD release amount at 96 h was 0.15 mg/g. But adsorption became more pronounced by 96 h, with COD release concentration decreasing from 39.8 mg/L to 12.0 mg/L, and NH4+-N release concentration dropping to 0 mg/L.
IFs and SS exhibited similar behaviors in terms of phosphorus and organic matter release, with their release concentrations increasing over time. Both materials contained relatively low phosphorus levels. Although IFs released phosphorus at a slightly higher concentration than SS, the actual amount released was negligible. Their COD release concentrations were significantly higher than those of CSC, and they showed potential for sustained release over time. After 96 h, IFs and SS released 0.20 mg/g and 0.06 mg/g of COD, respectively, suggesting significant amounts of impurities and water-soluble organic compounds—with IFs exhibiting the highest concentration and total release. The release of NH4+-N differed slightly: IFs reached its peak release concentration at 48 h before gradually decreasing, whereas SS reached its lowest release concentration at 48 h and then steadily rose. However, the actual NH4+-N release was minimal, reaching just 0.01 mg/g by 96 h.

3.3. Continuous Experiment on Domestic Sewage

3.3.1. Performance Comparison of Different Fe/C Fillers

The Fe/C filler demonstrated effective phosphate removal from high-concentration domestic sewage (Figure 4a). When the adsorption time was ≥48 h, the removal rate of all materials was no less than 93%. The optimal removal efficiency was achieved at Fe/C = 2:3 and HRT = 240 h, where the concentration decreased from 2.42 mg/L to 0.078 mg/L. After 168 h of contact, the COD removal rate exceeded 91% (Figure 4b). After 24 h, COD concentrations fell below 60 mg/L—hitting the most stringent Class A threshold under China’s rural wastewater treatment standards (DB53/T 953-2019 [35]). The sole outlier was the 2:1 iron-to-carbon ratio (Fe/C) group. Extending HRT to 48 h further reduced COD concentrations below 30 mg/L in most cases, meeting China’s Class IV Environmental Quality Standards for Surface Water (GB 3838-2002 [36]). Again, the Fe/C = 2:1 group slightly exceeded this threshold at 31 mg/L. In the NH4+-N adsorption experiment (Figure 4c), the adsorption capacity increased over time. At 24 h, 20~27% of the pollutants were adsorbed, and by 240 h, the ammonia nitrogen removal efficiency reached 90~98%. In summary: For Fe/C = 2:1, the material did not show an advantage in the early adsorption stage but exhibited sustained and stable adsorption capacity. For Fe/C = 2:3, the initial adsorption performance was slightly better than Fe/C = 2:1, but a rebound in COD levels was observed in later stages. The other two experimental groups showed good control over phosphorus and COD initially, but release phenomena occurred as time progressed.

3.3.2. Treatment Efficiency for Actual Wastewater Using Selected Fe/C Mixed Fillers

Both excessively high and low Fe/C ratios negatively impact treatment efficiency [37,38,39]. A low Fe/C ratio [38] means too much biochar, which disrupts surface area proportions, prevents corrosion layers from shedding, and slows down electrode reactions—ultimately reducing treatment effectiveness. Conversely, a high Fe/C ratio [39] leads to too few galvanic cells and clumped IFs, limiting the reactive surface area and compromising performance. To balance these trade-offs, a Fe/C material with optimal adsorption stability (Fe/C = 2:1) was selected for simulated experiments under actual operating conditions (designed HRT = 4.8 h).
During the 24-day continuous flow experiment, the filler demonstrated excellent and sustained stability in TP removal (Figure 5a). The influent TP concentration was 2.71 mg/L, with an effluent of 0.071 mg/L on the first day. By the sixth day, the effluent TP decreased to 0.044 mg/L, achieving a removal efficiency of 98.4%. Subsequently, the removal rate experienced a brief decline (98.0%) before steadily increasing to 99.4% by the 16th day. In the later stages, the removal efficiency remained above 98.9%, with an overall average TP removal rate of 98.6%. The filler also exhibited significant potential for COD removal (Figure 5b). The influent COD concentration was 65.2 mg/L, with the lowest effluent COD (0.8 mg/L) observed on the second day. In the later stages, the effluent COD fluctuated between 1.6 and 8.2 mg/L, meeting China’s Class I Environmental Quality Standards for Surface Water (GB 3838-2002). This resulted in an overall COD removal efficiency ranging from 87.4% to 98.8%, with an average of 92.8%. The Fe/C filler showed lower removal efficiency for TN in domestic wastewater compared to TP and COD (Figure 5c–e). In the first 10 days, the average TN removal rate was only 30.1%, but it gradually improved thereafter, reaching an overall TN removal efficiency of 40.9%. Based on the concentrations of NH4+-N and NO3−-N, the organic nitrogen (ON) content was estimated to be approximately 4~5 mg/L. In the initial 10 days, a significant portion of ON was degraded into NH4+-N, leading to poor NH4+-N removal performance (Figure 5d). Although Fe2+ acted as an electron donor and could reduce NO3− to NH4+ or convert it to N2 through indirect reduction by radicals, the oxidation rate of NH4+-N to NO3−-N exceeded the NO3−-N reduction rate. This explains why the effluent NO3−-N concentration was higher in the early stages. After 20 days, likely due to the near-formation of heterotrophic microbial communities, both ON degradation and denitrification rates improved, resulting in better TN and NO3−-N removal efficiencies.

3.4. Static Simulation Experiments for Phosphate Removal by Fillers Prepared at Different Temperatures and Ratios

During the 9-day static simulation experiments, Types I, IV, V, VI exhibited stable and sustained phosphate removal efficiency (Figure 6a). In contrast, the adsorption capacity of the other materials weakened after 72 h (Types II and VIII) or 96 h (Types III and VII), accompanied by increased desorption. Unprocessed bentonite exhibited low phosphorus adsorption capacity [40]. Although iron modification can enhance adsorption performance [41,42], no significant improvement was observed in this study, possibly due to the excessive bentonite dosage. When the mass ratio was reduced from 50% to 15%, the phosphorus removal efficiency exceeded 85%.
Types II, III, IV, VII, VIII exhibited poor removal efficiency for COD, likely due to the addition of bentonite, which may have led to the leaching of organic matter from the material itself and a reduction in adsorption capacity. Type I demonstrated the best performance, with COD levels decreasing to as low as 2.1 mg/L as adsorption time increased (Figure 6b). The micro-electrolysis process produces reactive hydrogen, ferrous ions, and ferric hydroxide in significant quantities. These byproducts help eliminate organic contaminants via adsorption and precipitation. Additionally, the process breaks down the complex ring and chain structures of stubborn organic molecules, transforming them into simpler forms that microbes can readily digest [4,43,44]. Types V and VI showed good adsorption performance, though their effectiveness varied with time: Type V’s adsorption capacity was positively correlated with time, while Type VI’s adsorption ability declined after 72 h. For Type IV, the maximum removal rate (79%) was achieved at 72 h, followed by a gradual release of organic matter.
Type IV achieved the highest TN removal efficiency, reaching 80% (120 h, 1.87 mg/L), but the concentration rapidly increased to 5.52 mg/L afterward. It also performed well in removing NH4+-N and NO3−-N. Types II and III initially displayed moderate adsorption capacity (removal rates of 20~38%), but after 168 h, the release of nitrogen-containing substances from the material itself caused an increase in total nitrogen concentration. Their NH4+-N removal rates generally exceeded 80%, but nitrate nitrogen treatment was ineffective. Types V~VIII also performed poorly in TN removal, with weak initial treatment efficiency and increased release effects later. NO3−-N was consistently in a state of release, while NH4+-N removal efficiency ranged from 21% to 94%. Although their performance for total nitrogen and NH4+-N was unsatisfactory, they showed better removal efficiency for NO3−-N (216 h, 97%). For Type I, the NO3−-N removal rate increased over time, while TN adsorption capacity gradually improved after 96 h, though ammonia nitrogen continued to be released.
It can be seen that the type of binder is a more critical performance determinant than the sintering temperature. Type I filler containing kaolin (sintered at 400 °C) shows the optimal and stable comprehensive pollutant removal ability, and also saves energy consumption compared with 800 °C.

4. Discussion

4.1. Performance of Fe/C Physical Mixture

This study demonstrates that Fe/C materials prepared through simple physical mixing exhibit outstanding removal efficiency for nitrogen, phosphorus, and organic matter in wastewater, representing an environmentally friendly material with straightforward operation, extremely low cost, and high potential for widespread application. Experimental results from basic physical mixing reveal that with an HRT of 24 h, over 80% of phosphates are removed—peaking at 98%. Extending the HRT to 96 h boosts ammonia nitrogen removal to over 72%, while COD removal tops out at 75%. Notably, the system excels at eliminating phosphates from high-strength domestic wastewater: at HRT durations of 48 h or longer, removal rates consistently exceed 93%. A higher iron-to-carbon ratio leads to more effective phosphate removal (e.g., 98% removal at Fe/C = 2:1), primarily attributed to electrochemically driven chemical precipitation. As the anode, IFs continuously release Fe2+ during micro-electrolysis, which is subsequently oxidized to Fe3+ and reacts with phosphate in the wastewater to form stable FePO4 or Fe3(PO4)2 precipitates [28]. Additionally, the generated iron (hydr) oxide colloids further capture phosphorus through surface adsorption and co-precipitation [27]. In contrast to total phosphorus (TP), chemical oxygen demand (COD) shows better treatment efficiency at a lower iron-to-carbon ratio (1:2). A lower Fe/C ratio produces more reactive hydrogen atoms and hydroxyl radicals, effectively degrading organic molecules via oxidation pathways. Conversely, a higher iron-to-carbon ratio (e.g., Fe/C = 3:1) may introduce substantial organic impurities attached to the IFs themselves, resulting in COD desorption outweighing adsorption (Table 2). For nitrogen removal, HRT plays a more dominant role, with lower Fe/C ratios demonstrating superior adsorption performance compared to higher ratios. This phenomenon likely occurs because iron depletes dissolved oxygen and hampers nitrification, leading to inefficient nitrogen removal [43].
Fe/C micro-electrolysis materials transform biochar’s surface properties and pore structure, creating additional active sites that boost their ability to adsorb nitrogen, phosphorus, and organic compounds. Research consistently supports these findings [30,45,46,47,48,49]: BET analysis shows iron coatings on biochar surfaces, which occupy minimal space but introduce irregular layers and large pores, improving adsorption. SEM images reveal jagged, smooth-surfaced aggregates with enhanced porosity. XPS data confirms that phosphorus adsorption involves ligand exchange alongside surface precipitation. FTIR spectroscopy highlights richer functional groups in modified biochar compared to untreated versions, while zeta potential measurements prove modified biochar is more electronegative.

4.2. Comparison of Physical Mixing and Preparation Methods for Filler Performance

In the preparation of Fe/C materials, physical mixing offers advantages such as low cost and simple operation, making it suitable for short-term or small-to-medium-scale pilot projects in rural areas with limited budgets and weak technical maintenance capabilities. However, long-term dynamic experiments in this study also reveal its potential risks: under prolonged operation and hydraulic scouring, physically mixed fillers are prone to Fe/C separation and compaction, which may reduce reaction interfaces and decrease efficiency.
High-temperature sintering (e.g., 800 °C) provides better stability but consumes more energy, and excessively high temperatures may damage the pore structure and reactivity of biochar. This study demonstrates that low-temperature sintering is more advantageous. Among the eight filler types, Type I (sintered at 400 °C) exhibits the most stable and comprehensive pollutant removal performance. Type I showed consistent and reliable phosphorus removal (82~97% efficiency). Over time, it reduced COD levels to as low as 2.1 mg/L, improved nitrate removal efficiency, and gradually enhanced total nitrogen adsorption after 96 h. Liu’s team combined iron scraps and biochar, then sintered them into spherical Fe/C materials at 300 °C for an hour, boosting COD removal [43]. When treated and blended at 450 °C, αMnO2 nanotubes and carbon nanotubes delivered top-notch oxygen reduction reaction (ORR) performance and long-term stability in alkaline conditions. Meanwhile, hydrothermally synthesized αMnO2 nanorods, annealed at 450 °C, displayed remarkable ORR activity, durability, and catalytic performance [50]. Zhao’s group created an innovative Fe/C substrate that adsorbed 7.78 mg/g of phosphorus within 24 h [39].

4.3. Effects of Relatively Low Temperature + Binder

One of the most innovative findings of this study is the validation of the feasibility of the “relatively low-temperature sintering (400 °C) + kaolin binder” approach. The Type I filler significantly reduces energy consumption while maintaining high performance, making it an ideal preparation solution. Natural kaolin is not a great binder due to its low CaO content, small surface area, and weak strength. But once calcined, it transforms into metakaolin—a material with superior mechanical and thermal properties, perfect for binding applications [51]. This method successfully avoids secondary pollution issues caused by the introduction of soluble organic matter, as seen with materials like bentonite [10]. Research indicates that this temperature range is sufficient to ensure the mechanical strength of the filler while maximizing the retention of biochar activity [11]. This process achieves an outstanding balance between cost, energy consumption, performance, and stability, paving the way for the large-scale and green application of Fe/C micro-electrolysis fillers in decentralized wastewater treatment [9].
This study introduces a novel Fe/C micro-electrolysis material prepared through a combination of physical mixing, kaolin binder, and low-temperature sintering at 400 °C. Composed of readily available raw materials, this material exhibits strong environmental sustainability. Experimental results indicated that this preparation method significantly enhances the long-term operational stability of the filler, making it a promising candidate for water treatment applications in resource-limited areas, such as rural regions. This work offers a practical and economically viable approach to developing efficient and eco-friendly water treatment materials.

5. Conclusions

(1) The Fe/C mass ratio of 2:1 was identified as the optimal proportion for treating rural domestic wastewater. Although did not show an advantage in the early adsorption stage but exhibited sustained and stable adsorption capacity. Static experiments showed that with this specific ratio, an HRT of 24 h achieved an impressive 98% phosphate removal rate. The physical mixed filler demonstrated stable removal rates of over 98.6% for TP and 92.8% for COD under dynamic continuous flow conditions, while TN removal rate can also reach 56.1% after 22 days. Phosphorus removal relied on chemical precipitation at high Fe/C ratios, while COD removal achieved better performance through micro-electrolysis oxidation at medium-to-low Fe/C ratios. In decentralized wastewater treatment facilities, incorporating an Fe/C micro-electrolysis unit can reduce both floor space and overall costs.
(2) SS is unsuitable as a replacement for IFs or iron powder. Although SS substitution for IFs exhibited excellent ammonia nitrogen removal (>93%), small-particle SS (3 mm) posed risks of phosphorus and COD release, making it less feasible than IFs.
(3) The approach of relatively low-temperature processing combined with kaolin clay binder can reduce energy consumption costs, offering a reliable technical option for decentralized rural wastewater treatment that is low-cost, high-efficiency, and easy to maintain.

Author Contributions

Conceptualization, J.S. and X.W.; methodology, X.R., D.W. and X.Z.; software, Y.L.; validation, S.S. and J.F.; formal analysis, Y.X.; investigation, X.R.; data curation, Y.L.; writing—original draft preparation, Y.X. and Y.L.; writing—review and editing, J.S.; visualization, Y.L.; supervision, X.W.; project administration, J.S., D.W. and X.Z.; funding acquisition, D.W. and X.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National key R&D projects of China (Grant No. 2024YFD1700100) and the project from Yunnan Provincial Communications Investment Construction Group Co., Ltd. (Grant No. YCIC-YF-2024-01).

Institutional Review Board Statement

No applicable.

Informed Consent Statement

No applicable.

Data Availability Statement

The datasets used and analyzed during the current study available from the corresponding author on reasonable request.

Conflicts of Interest

Authors Xiaojiao Ren, Xuejin Zhou and Di Wu were employed by the company Yunnan Comm. Investment Eco-Tech Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The authors declare that this study received funding from National key R&D projects of China, and the project from Yunnan Provincial Communications Investment Construction Group Co., Ltd. The funder was not involved in the study design, collection, analysis, and interpretation of data, the writing of this article or the decision to submit it for publication.

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Applsci 15 12355 g001
Figure 1. Continuous vertical flow device.
Figure 1. Continuous vertical flow device.
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Figure 2. Removal capacity of pollutants by IF–CSC with different ratios: (a) PO43−; (b) COD; (c) NH4+-N; (d) NO3−-N.
Figure 2. Removal capacity of pollutants by IF–CSC with different ratios: (a) PO43−; (b) COD; (c) NH4+-N; (d) NO3−-N.
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Figure 3. The removal capacity of pollutants by different SS–CSC ratios: (a) PO43−; (b) COD; (c) NH4+-N; (d) NO3−-N.
Figure 3. The removal capacity of pollutants by different SS–CSC ratios: (a) PO43−; (b) COD; (c) NH4+-N; (d) NO3−-N.
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Figure 4. Static adsorption capacity of Fe/C materials with different ratios for high-concentration wastewater: (a) PO43−; (b) COD; (c) NH4+-N.
Figure 4. Static adsorption capacity of Fe/C materials with different ratios for high-concentration wastewater: (a) PO43−; (b) COD; (c) NH4+-N.
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Figure 5. Continuous operation experiment for actual wastewater treatment: (a) TP; (b) COD; (c) TN; (d) NH4+-N; (e) NO3−-N.
Figure 5. Continuous operation experiment for actual wastewater treatment: (a) TP; (b) COD; (c) TN; (d) NH4+-N; (e) NO3−-N.
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Figure 6. Performance differences in Fe/C micro-electrolysis materials prepared by different methods: (a) PO43−; (b) COD; (c) TN; (d) NH4+-N; (e) NO3−-N.
Figure 6. Performance differences in Fe/C micro-electrolysis materials prepared by different methods: (a) PO43−; (b) COD; (c) TN; (d) NH4+-N; (e) NO3−-N.
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Table 1. Preparation of Fe/C Micro-electrolyte Materials.
Table 1. Preparation of Fe/C Micro-electrolyte Materials.
ParameterType IType IIType IIIType IVType VType VIType VIIType VIII
Iron (%)2010204010201020
Carbon (%)1020102020102010
Kaolin (%)30///5050//
Bentonite (%)/505025//5050
Ammonium Bicarbonate (%)101010/10101010
Quartz Sand (%)301010/10101010
Lime (%)///10////
Aluminum Powder (%)///5////
Sintering Temperature (°C)400400400Natural drying800800800800
Table 2. Release of N, P, and COD from materials.
Table 2. Release of N, P, and COD from materials.
IndicatorTreatment Time (h)CSCIFSS
PO43− (mg/L)244.310.040.02
484.430.060.02
965.600.070.02
COD (mg/L)2417.1053.2046.40
4839.8044.5049.00
9612.00110.6052.80
NH4+-N (mg/L)240.053.422.52
480.116.130.30
960.004.295.32
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Xu, Y.; Ren, X.; Wu, D.; Zhou, X.; Liu, Y.; Sun, S.; Feng, J.; Shen, J.; Wang, X. Performance Evaluation of Readily Available Iron–Carbon Micro-Electrolysis Materials for Domestic Sewage Treatment. Appl. Sci. 2025, 15, 12355. https://doi.org/10.3390/app152312355

AMA Style

Xu Y, Ren X, Wu D, Zhou X, Liu Y, Sun S, Feng J, Shen J, Wang X. Performance Evaluation of Readily Available Iron–Carbon Micro-Electrolysis Materials for Domestic Sewage Treatment. Applied Sciences. 2025; 15(23):12355. https://doi.org/10.3390/app152312355

Chicago/Turabian Style

Xu, Yong, Xiaojiao Ren, Di Wu, Xuejin Zhou, Yanping Liu, Shanshan Sun, Jimeng Feng, Jian Shen, and Xinze Wang. 2025. "Performance Evaluation of Readily Available Iron–Carbon Micro-Electrolysis Materials for Domestic Sewage Treatment" Applied Sciences 15, no. 23: 12355. https://doi.org/10.3390/app152312355

APA Style

Xu, Y., Ren, X., Wu, D., Zhou, X., Liu, Y., Sun, S., Feng, J., Shen, J., & Wang, X. (2025). Performance Evaluation of Readily Available Iron–Carbon Micro-Electrolysis Materials for Domestic Sewage Treatment. Applied Sciences, 15(23), 12355. https://doi.org/10.3390/app152312355

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