A three-dimensional electrochemical oxidation system with α-Fe2O3/PAC as the particle electrode for ammonium nitrogen wastewater treatment

A three-dimensional particle electrode loaded with α-Fe2O3 on powdered activated carbon (PAC) (α-Fe2O3/PAC) was synthesized by the microwave method for removing ammonium nitrogen from wastewater in a three-dimensional electrode system. The α-Fe2O3/PAC electrode was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The effect of the added α-Fe2O3/PAC on the removal of ammonium nitrogen from simulated wastewater was studied by changing the cell voltage, particle dosage, and particle electrode synthesis conditions. Simulated experiments were also carried out on different pollutants under the best experimental conditions and the actual domestic sewage was tested. The results show that the optimal synthesis conditions of the particle electrode are as follows: the ratio of PAC to anhydrous FeCl3 is 1 : 2, and the microwave power is 1000 W for 60 s. After 20 min of electrolysis at 20 V, the ammonium nitrogen removal rate can reach 95.30%.


Introduction
Domestic sewage contains many pollutants, such as organic matter, phosphate, bacteria and ammonium nitrogen (NH 4 + -N). 1 Particularly, as a signicant nutrient, ammonium nitrogen can lead to eutrophication in polluted water when in excess. A high nutrient load gives rise to detrimental effects such as the rapid growth of harmful algae in contaminated water, reduced dissolved oxygen and declining water self-purication ability and can even compromise the structure and function of aquatic ecosystems. [2][3][4][5] Hence, for reducing the negative impact of wastewater discharge, various traditional treatment methods have been studied to remove the ammonium nitrogen from polluted water, including biological treatments 6 and diverse approaches of physical and chemical processes, such as adsorption, 7 membrane ltration, 8 ion exchange, 9 air stripping, 10 breakpoint chlorination 11 and chemical precipitation. 12 Electrochemical oxidation processes have been considered as environment friendly technologies in water treatment processes due to their strong oxidation properties, ease of control and mild reaction conditions. 13,14 In effect, electrochemical technologies provide a second solution to numerous environmental issues in industrial processes because of electrons, which provide readily automatable, highly effective, economical, clean and multifunctional reagents. 15 Threedimensional (3D) electrodes have received wide attention due to the large specic surface area and high conversion rate compared with the conventional two-dimensional (2D) system. The reason for this phenomenon is that the tiny particles placed in the 3D electrode system create charged microelectrodes on account of the action of an electric eld. 16,17 Because of the large specic surface area of these particles, the 3D electrode system can adsorb contaminants, enhance the conductivity and even provide more active sites for the catalytic reaction, thereby increasing the removal efficiency. 18 In the recent years, various conventional types of particle electrodes have been widely studied, including granular activated carbon, carbon aerogels, modied kaolin and metal particles. 17,[19][20][21] Iron oxide exists in many forms in nature, among which magnetite (Fe 3 O 4 ), maghemite (g-Fe 2 O 3 ) and hematite (a-Fe 2 O 3 ) are probably the most common and technically important. 22 Iron oxides have been reported to have important applications in the removal of ammonium nitrogen. Zare et al. used Fe 3 O 4 nanoparticles as adsorbents to remove ammonium ions from simulated wastewater. Aer 40 min of adsorption, the ammonium nitrogen removal rate was 94.3%. 23 Wu et al. lled metal oxide (CuO, MnO 2 and Fe 2 O 3 )-supported GAC as a particle electrode and studied the ability of the 3DER-3DBER system to eliminate nitrogen contaminants. 24 Liu et al. achieved an ammonium removal rate of 89.97% aer adsorption for 2 hours using magnetic zeolite NaA with 3.4% Fe 3 O 4 loading. 25 In addition, AC has been applied as a carrier material because of its excellent adsorption ability, porous structure and mechanical strength. 26 Therefore, the purpose of this study was to synthesize a modied powdered activated carbon particle electrode loaded with a-Fe 2 O 3 (a-Fe 2 O 3 /PAC) by the microwave method in one step, characterize the a-Fe 2 O 3 /PAC electrode, and analyze how the synthesized a-Fe 2 O 3 /PAC as the particle electrode participates in the degradation of NH 4 + -N in simulated wastewater.

Preparation of particle electrodes
PAC and anhydrous FeCl 3 were mixed in different mass ratios and ground uniformly using an agate mortar. The mixture transferred from the mortar to a crucible was subjected to a microwave reaction in a household microwave oven (Microwave Oven, Panasonic NN-GF352M, 2450 MHz, 1000 W) with different heating times and microwave powers. Aer cooling for 1 hour, the obtained particle electrodes were stored in glass bottles. The synthesis ow chart is shown in Fig. 1. Table 2 shows the different experimental conditions for preparing the a-Fe 2 O 3 /PAC particle electrodes.

Experimental procedure
The experimental device of the 3D electrode system is shown in Fig. 2. 3D electrochemical oxidation experiments were performed in a heat-resistant glass tank with an effective volume of 85 mL. Ti/RuO 2 -IrO 2 electrode (25 mm Â 20 mm Â 1 mm) (Baoji Jinbu Titanium Nickel Equipment Co. Ltd) and stainless steel plate (25 mm Â 20 mm Â 1 mm) were employed as the anode and cathode. The two plates were parallel to each other with a spacing of 2 cm. The power source used in the experiment was regulated DC power supply (APS3005DM, ATTEN Instrument, China). A certain mass of the particle electrode was added to the system for electrolysis experiment under the corresponding voltage. All the experiments were performed at room temperature, and 2 mL water samples were taken from the system every 10 min for testing the corresponding indicators.  (1):

Analysis
Here, C 0 (mg L À1 ) is the initial concentration of the pollutant, and C t (mg L À1 ) is the concentration of the pollutant at time t.

Characterization of a-Fe 2 O 3 /PAC particle electrode
The characteristics of the synthesized a-Fe 2 O 3 /PAC particle electrode were investigated by SEM and XRD. Fig. 3 presents the SEM images of the particle electrode magnied 3000 times. The SEM image in Fig. 3(a) shows the morphology of PAC. The surface of the PAC particles exhibits a relatively smooth state. As shown in Fig. 3(b-d), the materials grown on the surface of the PAC particles are approximately spherical and irregularly polyhedral due to the loading of a-Fe 2 O 3 . Different synthesis conditions may result in different particle morphologies of the samples. The material shown in Fig. 3(b) displays good regularity and crystallinity in the crystal structure of a-Fe 2 O 3 (sample b). The samples depicted in Fig. 3(c and d) (sample a and sample f) show poor crystallinity and insufficient loading on PAC as a result of unsuitable material synthesis conditions. As shown in Fig. 4, the synthesized samples of the a-Fe 2 O 3 /PAC materials (sample b) under optimal synthesis conditions and PAC have been characterized by XRD. The XRD pattern of PAC in Fig. 4 shows that PAC exhibits two dispersion peaks. One of them is a signicantly broad peak at approximately 2q ¼ 23 , which also appears in the diffraction pattern of the synthesized sample (b). The diffraction peaks of a-Fe 2 O 3 /PAC were consistent with the standard hematite (a-Fe 2 O 3 ) pattern (PDF#87-1164). The main peaks were located at 2q ¼ 24. 16

Effect of cell voltage
The chloride ions in water are oxidized to Cl 2 molecules at the anode, which then dissolve in water to form HClO. HClO reacts with NH 4 + to form N 2 . 11 The main reactions are shown in the following equations: 27     This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 8773-8779 | 8775 (>95%) no longer increased considerably. At the appropriate voltage, particles can be polarized to form a myriad of microelectrodes, which can enhance the oxidation and increase the rate of the above-mentioned reactions. 18 As the cell voltage increases, the number of polarized particles rise, which leads to an enhancement in the rate of oxidation and improvement in the rate of degradation. In addition, aer 20 min of electrolysis using 20 V and 25 V as the battery voltage, the NH 4 + -N removal efficiency exceeded 95%. However, a very high voltage will cause an increase in the system temperature, resulting in heat loss and energy waste.

Effect of the amount of a-Fe 2 O 3 /PAC particle electrode
The change in the NH 4 + -N degradation rate at different dosage of the a-Fe 2 O 3 /PAC particles (voltage ¼ 20 V) is shown in Fig. 5(b). The increase in the amount of a-Fe 2 O 3 /PAC added led to an increase in the rate of degradation. This phenomenon became apparent aer 10 min of electrolysis and aer 30 min, the degradation curve aer adding 0.4 g a-Fe 2 O 3 /PAC particles rst started to stabilize, at which time the electrolysis was saturated. However, at the same time, the removal efficiencies aer adding 0.2 g and 0.3 g a-Fe 2 O 3 /PAC particles were 81.89% and 90.00%, respectively. When a certain voltage was applied to both sides of the electrolytic cell, each a-Fe 2 O 3 /PAC particle electrode in the cell was polarized and charged and served as the anode and cathode of the microelectrolytic cell. Moreover, the adsorption of activated carbon makes oxidation more likely to occur. As the number of added   particle electrodes increased, more charged microelectrodes were formed, resulting in an increase in electrolysis efficiency. [28][29][30] Furthermore, aer 40 min of electrolysis, the NH 4 + -N removal efficiency was as high as 95% aer adding 0.3 g and 0.4 g a-Fe 2 O 3 / PAC particles. At this point, the increase in dosage will not lead to a signicant increase in the efficiency of degradation.

Effect of preparation conditions of particle electrode
The effects of different synthesis conditions of particle electrodes on NH 4 + -N removal in a 3D electrode system are shown in Fig. 5(c and d).
A large proportion of FeCl 3 in the synthesis process led to better degradation (Fig. 5(c)). Aer 20 min of electrolysis, the electrolysis efficiency of the experiment using samples (b) and (c) reached 95%, showing a signicantly higher level than that for sample (a) (75.08%) and sample (d) (70.96%). This phenomenon can be explained by the fact that as the content of Fe 2 O 3 in the synthesized particle electrodes increases, a-Fe 2 O 3 /PAC can form more micro-electrolytic cells, resulting in a larger specic surface area and more efficient mass transfer distance. The smaller distance between the anode and cathode in each microelectrolysis cell is more benecial for the transfer of reagents between the electrode surfaces. 28 Inevitably, more Fe 2 O 3 will lead to the dissolution of more iron ions to some extent. The possibility of the formation of iron hydroxide rises, which will improve the removal efficiency of NH 4 + -N in cooperation with threedimensional electrolysis. The specic synthesis conditions are shown for the samples (a-d) in Table 2. In addition, the effect of the particle electrodes reacted at different microwave powers and times on the degradation rate of NH 4 + -N is not much different, and sample (b) shows the best effect (Fig. 5(d)). Short microwave heating times and inadequate microwave power may result in incomplete material synthesis. The SEM image shown in Fig. 3(c and d) indicates insufficient Fe 2 O 3 supported on the activated carbon. Table 3 shows the degradation of NH 4 + -N aer electrolysis for 20 min at 20 V under different particle electrode preparation conditions.

Experiment under optimal electrolysis conditions
With sample (b) as the particle electrode and a voltage of 20 V, a 3D electrolysis experiment was carried out, and NH 4 + -N, TN and TP were studied under 2D and 3D system conditions using simulated wastewater. It can be clearly observed in Fig. 6 and 7 that the NH 4 + -N, TN and TP removal efficiency in the 2D system rises slowly with the increase in time and in the 3D system, it rapidly rises rst and then maintains a smooth state. In particular, PAC was used as the particle electrode for the electrolytic testing of NH 4 + -N under the same 3D conditions (3D-P). The results showed that under the inuence of activated carbon adsorption, low impedance and weak catalytic performance, the degradation curve increased tortuously, and the removal efficiency of NH 4 + -N aer 60 min of electrolysis was only 26.27%. The degradation of NH 4 + -N (Fig. 6) and TN ( Fig. 7(a)) increased rapidly in the rst 20 min and then did not increase signicantly. The degradation efficiency of NH 4 + -N was 95.30% in the 3D system but only 34.73% in the 2D system aer treatment for 20 min. In the same case, the removal efficiencies of TN were 67.97% (3D) and 19.47% (2D). Besides, TP degraded in 10 min (99.59%) in the 3D system, and only 5.15% TP degraded in the 2D system ( Fig. 7(b)). The oxidation and removal of ammonium nitrogen by HClO during the electrolysis process play leading roles in this study. The main mechanism of ammonium nitrogen degradation is the secondary oxide produced by anodic electrolysis to remove NH 4 + -N. 31,32 However, a small amount of iron hydroxide precipitated during the electrolysis process also plays a role in  the removal of ammonium nitrogen. The reason behind this phenomenon is that when the ammonium nitrogen removal effect is remarkable, the amounts of the oxidizing substance (HClO) and H + in the solution are large, resulting in a decrease in the pH of the solution. 33 At the same time, a small amount of an electrolyzed water side reaction may occur during electrolysis. It has been shown in the literature that at pH < 5.6, the dominant functional group on the surface of iron oxide is Fe 2+ or FeOH + , and iron oxide attracts anions at low pH. When the ambient pH is lower than the pH PZC (6.7) of hematite, anions are expected to adsorb on the surface of the positively charged hematite by electrostatic attraction. [34][35][36] The above-mentioned explanation can be regarded as one of the reasons why TP can be removed in large quantities.

Effect of actual domestic wastewater degradation
The experimental system can obtain a large NH 4 + -N removal rate in a short time (20 min) under optimal conditions, and its application for the actual domestic sewage is also effective. Based on the exploration of the above-mentioned experiments, an electrolysis experiment was carried out on the domestic sewage of a factory. The detailed degradation efficiency data under the three-dimensional system are shown in Table 4. Due to the complexity of the components in the actual sewage, the oxidizing substances in the water are more complicated than that in the simulated wastewater. Therefore, aer 20 min of electrolytic treatment, the degradation effect of NH 4 + -N in the sewage can reach 98.97%. Meanwhile, TN, TP and COD in the wastewater were also tested. It was found that the removal rate of TN was 91.11% aer 20 min, and TP achieved good results aer 10 min with a removal rate of 99.75%. COD also had a removal rate of 52.88% aer 40 min of electrolysis. Compared with other studies for the removal of NH 4 + -N, this study can achieve a higher ammonium nitrogen removal rate in a shorter period of time (Table 5).
Since the particle electrode used in this study was synthesized with PAC as the carrier, the separation of PAC in practical applications is also a crucial topic that needs to be discussed. According to recent research, several separation processes have been widely used in practical plants, such as sedimentation, membrane ltration, ultraltration and pile cloth ltration. It has been reported that occulation may have given PAC a better settling characteristic with the addition of iron. 37 For this reason, a-Fe 2 O 3 /PAC can be separated by sedimentation in practical applications. However, the hydraulic retention time required for sedimentation needs to be greater than 120 min. Thus, in practical applications, combining precipitation with membrane ltration (backwash interval ¼ 10-20 minutes) can be considered to improve the processing efficiency. 38

Conclusion
It is benecial to use a-Fe 2 O 3 /PAC as a 3D particle electrode for the electrochemical oxidation of ammonium nitrogen. The removal rate of ammonium nitrogen in the electrolysis for 20 min is as high as 95.30% from the simulated wastewater under the optimal conditions (an applied voltage of 20 V, an a-Fe 2 O 3 /PAC dosage of 0.3 g (PAC : FeCl 3 ¼ 1 : 2, microwave power ¼ 1000 W, time ¼ 60 s)). Under identical conditions, aer the domestic sewage was tested, the removal efficiencies of NH 4 + -N and TN were found to be 98.97% and 91.11%, respectively, at the end of 20 min. Moreover, TP removal could reach 99.87% in 10 min. Consequently, the 3D electrochemical oxidation with the a-Fe 2 O 3 /PAC particle electrode is a prospective process for NH 4 + -N wastewater treatment.

Conflicts of interest
There are no conicts to declare.