Coupled Electrolysis–Microfiltration System for Efficient Phosphorus Removal and Recovery in the Form of Iron Phosphate Compounds from Wastewater

Abstract

Electrochemical technology presents a promising approach for phosphorus recovery from wastewater. Nevertheless, its application in industry is hindered by relatively low phosphorus recovery efficiency, high energy consumption and complex reactor configurations. In this study, a coupled electrolysis and microfiltration system was designed for phosphorus recovery in the shape of iron phosphate compounds with the use of steel pickling wastewater as the iron source. In the electrolysis unit, the anode diffusion layer was extracted from the porous anode surface with the production of an acid effluent and an alkaline effluent. The alkaline effluent was mixed with the stainless steel acid washing wastewater generated from the steel pickling process and then introduced into the microfiltration unit to intercept the iron phosphate crystals. The filtered effluent was finally introduced into the air aeration unit to further reduce the phosphorus content in the water. And the extracted acid solution could be reused in the pickling step of the iron and steel manufacturing process. The experimental results show that the coupled system achieved phosphorus recovery of 42~80% at a current density of 5~20 mA cm⁻², accompanying energy consumption of 5.78~9.15 kWh (kg P)⁻¹ and current efficiency of 79~43%, when the phosphorus concentration was 3 mM and the iron–phosphorus molar ratio was 1.5. After the microfiltration treatment, the residual phosphorus could be further reduced to 0.5 mg L⁻¹ within 30 min at an aeration rate of 80 mL min⁻¹, which met the discharge standard. The presence of interfering ions (HCO₃⁻ and SiO₄²⁻) posed inhibited effects on phosphorus recovery. Generally, this study provides a green and environmentally friendly way to efficiently recover phosphorus resources from wastewater.

1. Introduction

Phosphorus is a necessary element for biological growth, but also a non-renewable resource. At the same time, massive amounts of phosphorus in industrial wastewater and domestic sewage have been discharged into the natural environment, causing water eutrophication [1,2]. If these phosphorus resources were to be recycled, it would greatly alleviate the current phosphorus crisis. Currently, alkaline crystallization is a common method of phosphorus removal and phosphorus recovery from wastewater in the forms of insoluble compounds such as struvite or calcium phosphate. But this crystallization method requires the addition of alkaline and acid reagents to adjust the solution to 8–11 to precipitate the phosphorus-containing compounds and neutralize the water, which inevitably leads to secondary pollution and increases the salinity [3,4].

Electrochemical precipitation, as an emerging technology for phosphorus removal, has a wide range of application prospects, with no need for massive chemicals inputs and no increase in the water salinity. During the electrolysis process, the H₂O-splitting reaction at the cathode produces OH⁻ in the cathode diffusion layer, thereby creating a highly alkaline environment and prompting insoluble phosphorus-containing compounds’ precipitation at the cathode surface [5]. But the phosphorus recovery efficiency is greatly limited by the mass transfer of lattice ions toward the cathode, resulting in lower phosphorus recovery efficiency and higher energy consumption [5]. Recently, some divided electrolytic cells were developed to increase the OH⁻ utilization for phosphorus recovery [6]. For example, Zhu et al. developed an electrochemically mediated precipitation microfiltration system with the incorporation of a polytetrafluoroethylene (PTFE) membrane to achieve the efficient separation of acids and bases [6]. So, insoluble phosphorus-containing compounds could precipitate in the alkaline solution without the effect of mass transfer. But the used porous membrane greatly suffered from some problems such as membrane fouling, high resistance and complex reactor design.

Vivianite (Fe₃(PO₄)₂·8H₂O) is one of the typical forms of insoluble phosphorus-containing compounds and is mainly distributed in phosphorus-rich sedimentary iron ore and peat bogs. At the same time, vivianite can be also found in anaerobic sedimentary environments [7,8]. As compared with other recoverable phosphate products, the vivianite crystallization method following the reaction in Equation (1) is a more profitable route to recover phosphorus from wastewater and exhibits wider application prospects. Specifically, Rao et al. used vivianite to prepare lithium ferrous phosphate batteries, which have higher stability, a smooth charging and discharging plateau, good electrochemistry and cycling and a longer service life [9]. The formation of vivianite can occur under mild conditions with the relatively high purity of phosphorus, and the performance of phosphorus recovery in the form of vivianite is mainly affected by the solution pH, oxidation reduction potential (ORP), Fe/P molar ratio, etc. [10,11]. The electrocoagulation process has been widely implemented to recover phosphorus with the in situ generation of Fe²⁺ [12,13]. Zhang et al. realized phosphorus recovery by the electrochemical in situ generation of Fe(II) to form vivianite under low dissolved oxygen conditions [12]. However, the method suffers from poor long-term operational stability and high operating costs, owing to the continual consumption of iron anodes. Thus, it is attractive to use waste iron resources to recover phosphorus from wastewater to realize the goal of “treating waste with waste”.

3Fe²⁺ + 2PO₄³⁻ → Fe₃(PO⁴)₂

(1)

In this study, an efficient phosphorus recovery system was developed by combining a novel divided electrolyzer with a microfiltration device. In the electrolysis system, the basic solution was produced and was mixed with steel pickling wastewater to obtain the vivianite precipitation, and finally, solid–liquid separation was realized by a filtration system. In addition, the effects of different electrochemical conditions and water matrixes on phosphorus recovery were systematically investigated in this study. The performance of this electrolytic system for phosphorus recovery from wastewater was systematically investigated. Finally, the effects of coexisting ions (HCO₃⁻ and SiO₄²⁻) on the phosphorus recovery performance were explored in the coupled system, and the products were characterized by XRD and SEM. This study provides some references for the design of energy-efficient electrochemical systems for phosphorus recovery.

2. Materials and Methods

2.1. Experimental Equipment

In this study, all materials and reagents are provided in Text S1. A membrane-free electrolyzer was developed to extract H⁺ from the anode surface, in which a 304 stainless steel tube cathode (diameter 80 mm, length 200 mm) and a tubular titanium porous anode (pore size 5 μm, diameter 40 mm, length 200 mm) were placed concentrically in a 1.0 L Plexiglas cylinder at a distance of 20 mm. The tubular Ti filters in electrolysis cells were both coated with a RuO₂-IrO₂-SnO₂-Sb₂O₅ mixed catalyst layer using the Pechini method reported in our previous study [14]. As shown in Figure 1, phosphorus-containing solution was pumped in from the bottom of the electrolytic cell by a peristaltic pump and pumped out at the top. Meanwhile, the alkaline solution was mixed with the pickling water in the microfiltration unit to retain the phosphorus-containing precipitates. Then, the residual Fe²⁺ in the filtrate was oxidized in the aeration unit to further reduce the phosphorus content. During operation, a peristaltic pump pumped the electrolyte from the bottom into the reactor at an influent flow rate of 500 mL min⁻¹. Under the conditions of a current density of 10 mA cm⁻², an influent pH of 8.0, and an initial phosphorus concentration of 3 mM, the electrochemically produced alkaline solution was mixed with the pickling wastewater at the Fe/P molar ratio of 1.5, and then introduced into the subsequent unit for phosphorus removal after mixing. All experiments were performed at room temperature and conducted repeatedly.

To mimic the typical composition of the phosphorus-containing wastewater generated in the electroplating industry, the synthesized solution consisted of 3 mM K₂HPO₄ and 10 mM Na₂SO₄. The solution conductivity was near 2.0 mS cm⁻¹ and the initial solution pH was about 8.0. The steel pickling wastewater was supplied by Shandong Wanfang Metal Materials Co., and the main ion concentrations and chemical indexes are shown in

Table 1. Quality parameters of the acid washing wastewater.

Parameters	Concentration
Fe2+	2.3 mol/L
Total Fe	2.5 mol/L
pH	~0
Conductivity	10.2 mS cm−1

, which were mixed with alkaline solution produced by the electrolytic cell with a set molar ratio of Fe to P.

2.2. Characterizations and Analytical Methods

The morphology of the collected precipitates was examined using scanning electron microscopy (SEM, Zeiss Sigma, Germany). The crystal structure of the precipitates was analyzed by X-ray diffraction (XRD, Rigaku Ultima IV powder X-ray diffractometer, Japan). The ammonium molybdate spectrophotometric method was used to determine the phosphorus concentration in solution [15]. The determination of Fe(II) and total iron concentration in solution was conducted based on a 1,10-o-phenanthroline spectrophotometric colorimetric assay [16]. And the acid–base titration method was used to determine the concentration of HCO₃⁻ [17]. A multiparameter water quality analyzer (Leici DZS708, Shanghai, China) was used to survey the solution pH and conductivity.

Geochemical Equilibrium Calculations. Visual MINTEQ 3.1 was used to perform equilibrium calculations that evaluated the saturation state of the solutions with respect to possible P-containing solids [18]. The saturation index was defined as follows:

$$SI={\displaystyle \frac{IAP}{{\mathrm{K}}_{sp}}}$$

(2)

where IAP is the ion activity product and K_sp is the solubility product of the precipitate phase. The solution is considered to be unsaturated when the SI is below zero, at saturation when it is zero and supersaturated when the SI is above zero.

The P recovery efficiency was calculated as follows:

$$R\left(\%\right)={\displaystyle \frac{C_{in}-C_{out}}{C_{in}}}$$

(3)

where R is the P recovery efficiency (%); C_in and C_out are the P concentrations (mM) in the influent and effluent, respectively.

The P recovery rate was calculated as follows:

$$J={\displaystyle \frac{{(C}_{in}-C_{out})\mathrm{V}}{\mathrm{A}\mathrm{T}}}$$

(4)

where J is the P recovery rate (g P (m² h)⁻¹); C_in and C_out are the P concentrations in the influent and effluent, respectively (g P L⁻¹); V is the volume of the sample (L); A is the electrode area (m²); T is the time of sample collection (h).

3. Results

3.1. Effect of Water Quality Parameters on P Recovery

3.1.1. Initial pH

The pH of the solution fundamentally affects the ionic interactions, rearrangements and aggregation, leading to changes in crystal morphology [19,20]. In order to investigate the effect of the initial pH of wastewater on phosphorus recovery, the phosphorus recovery in the electrolytic cell with anode diffusion-layer separation was evaluated at an initial pH ranging from 7 to 9. As shown in Figure 2a, species distribution results calculated by Visual MINTEQ show that nearly 99.5% of Fe was present as Fe²⁺ and FeHPO₄ in the initial pH range of 7.0~9.0. At pH 7, the phosphorus species was dominated by H₂PO₄⁻ and HPO₄²⁻. With further increases in pH, H₂PO₄⁻ transformed into HPO₄²⁻ via the deprotonation reaction, and therefore, the phosphorus species existed mainly in the form of HPO₄²⁻. In addition, the formation of vivianite and ferrous hydroxide depends on the degree of supersaturation and formation kinetics [12]. It is known that the condensation of ferrous hydroxide is highly sensitive to pH and is easily transformed into ferric hydroxide in the presence of oxygen through an oxidative mechanism. As shown in Figure 2b, the calculated saturation index (SI) values of iron phosphate compounds as a function of pH by Visual MINTEQ show that the mixture of pickling waste solution and the alkaline solutions discharged from the electrolytic reactor with anode diffusion-layer separation was preferentially saturated with vivianite rather than ferric hydroxide formation when the experimental pH varied in the range of 6–9 [21].

In addition, as shown in Figure 2, the electrolytic cell with an anode diffusion layer coupled with the microfiltration unit recovered about 39% of phosphorus at pH₀ = 7, while 80% phosphorus recovery was achieved at an initial pH = 8. Figure 2c shows that when the initial pH was increased from 7.0 to 8.0, the pH of the alkaline water increased from 6.0 to 6.5 after the addition of the pickling wastewater with the Fe/P molar ratio of 1:1.5, and the pH of the alkaline water only increased to 6.7 with a further increase in the initial pH to 9.0. Thus, when the initial pH of the electrolysis feed water increased from 7.0 to 8.0, the phosphorus recovery increased sharply from 39% to 80% and iron recovery increased from 21% to 33%, with the corresponding energy consumption decreasing from 20.17 kWh (kg P)⁻¹ to 9.15 kWh (kg P)⁻¹ (Figure 2d). The results of further increasing the initial pH of the solution to 9.0 showed that the phosphorus and iron recovery efficiencies were increased slightly to 85% and 39%, with energy consumption reducing to 8.33 kWh (kg P)⁻¹. The reason for this small increase may be that the alkaline effluent from the electrolysis reactor could reach a pH of 11.5 at an initial pH of 8.0, and the solution pH can still be higher than 6.5 by mixing the acidic ferrous waste solution, which was sufficient for the precipitation of vivianite. Therefore, the phosphorus recovery efficiency did not increase significantly, even if the initial pH was further increased to 9.0. In summary, relatively high phosphorus can be recovered in the form of vivianite at pH₀ > 8.0 with relatively low energy consumption.

3.1.2. Initial Phosphorus Concentration

It is well known that phosphorus species in solution exist in the main form of HPO₄²⁻ and H₂PO₄⁻ at solution pH = 8, and this shows the buffering capacity, since these two phosphorus species transform into each other as the pH changes under nearly neutral conditions [22]. Therefore, the buffering capacity of the solution increased and the pH change extent decreased when the phosphorus concentration increased. In order to investigate the influence of the initial phosphorus concentration on the phosphorus recovery, the changes in solution pH and phosphorus recovery efficiency after electrochemical treatment were evaluated under conditions with the initial phosphorus concentration ranging from 1 to 5 mM. As shown in Figure 3, the alkaline effluent pH from the electrolytic cell decreased from 11.6 to 11.3 with the increase in the initial phosphorus concentration from 1 to 5 mM, and the pH of the solution decreased from 6.7 to 5.7 after mixing with the pickling wastewater under the experimental conditions of a Fe/P molar ratio of 1.5, an initial pH of 8.0 and a current density of 10 mA cm⁻² (Figure 3a). Meanwhile, the phosphorus recovery efficiency was 86% and 80% at initial phosphorus concentrations of 1 mM and 3 mM, respectively. These results were reasonable since the optimal pH range for vivianite precipitation was from 6.0 to 9.0 [23]. But the pH of the solution mixed with alkaline effluent from the electrolysis reactor and the acid pickling wastewater was lower than 6.0, owing to the strong buffering effect. Thus, phosphorus recovery of only 41% was achieved after further increasing the phosphorus concentration to 5 mM (Figure 3b). But it can be noted that the recovery energy consumption of 24.3 kWh (kg P)⁻¹ at the initial phosphorus concentration of 1 mM was higher than that at initial phosphorus concentrations of 3 mM and 5 mM.

3.1.3. Fe/P Molar Ratio

In order to investigate the effect of the Fe/P molar ratio on the phosphorus recovery, the performance of the coupled processes was evaluated at Fe/P molar ratio ranging from 1.5 to 2.5 under the experimental conditions of phosphorus-containing water with an initial pH of 8, initial phosphorus concentration of 3 mM and a current density of 10 mA cm⁻². As shown in Figure 4a, the alkaline effluent from the electrolysis reactor with pH of 11.5 was mixed with the pickling wastewater, and the pH values of the mixed wastewater were 6.6, 6.1 and 5.5, respectively at Fe/P molar ratio of 1.5, 2.0 and 2.5. The phosphorus recovery could reach more than 80% (Figure 4b) with energy consumption as low as 9.1 kWh (kg P)⁻¹ at an Fe/P molar ratio of 1.5 and 2.0. However, when the Fe/P molar ratio was increased to 2.5, the solution pH decreased to only 5.5 after mixing the alkaline effluent from the electrolysis reactor and the acid pickling wastewater, decreasing the phosphorus recovery efficiency to 72% and increasing energy consumption to 10.1 kWh (kg P)⁻¹ [24]. It is known that an increase in the Fe/P molar ratio shall inhibit the reaction of Fe²⁺ with OH⁻ in water and favor the complexation reaction for vivianite precipitation. However, the Fe²⁺ source was provided by the acid pickling wastewater, and increasing the Fe/P molar ratio meant the simultaneous increase in the addition of H⁺, which made the solution pH decrease after mixing, which was unfavorable to the saturation precipitation of vivianite [25]. Therefore, the Fe/P molar ratio of 1.5 was the most favorable for phosphorus recovery under the present studied conditions.

3.2. Effect of Current Density on Phosphorus Recovery

In the coupled electrolysis–microfiltration system, the current density plays a decisive role in the phosphorus recovery efficiency. Therefore, the effect of current density was systematically studied in relation to the pH of the effluent from the electrolysis reactor, as well as the phosphorus recovery efficiency, energy consumption and current efficiency. As shown in Figure 5a, the alkaline effluent pH from the electrolysis reactor increased from 11.0 to 11.8 as the current density rose from 5 to 20 mA cm⁻², while the acidic effluent pH correspondingly reduced from 2.3 to 1.7. This indicates that the increase in current density promoted the water electrolysis reaction at the electrodes, resulting in an enhanced yield of H⁺ and OH⁻ [26]. The increase in alkaline effluent pH from the electrolysis reactor at high current density showed a relatively strong buffering effect to reduce the extent to which pH decreased to when mixing with the acid pickling wastewater. Thus, after adding the iron-containing acid solution into the alkaline effluent at a molar ratio of iron to phosphorus of 1.5, the solution pH reduced from 5.0 to 7.0 with elevating current density from 5 to 20 mA cm⁻². Specifically, the solution pH increased rapidly from 5.0 to 6.5, and the phosphorus and iron recovery efficiencies increased dramatically from 42% to 80% and from 20% to 35%, respectively, when the current density increased in the range of 5~10 mA cm⁻², (Figure 5b), corresponding to an increase in energy consumption from 5.78 to 9.15 kWh (kg P)⁻¹. As the current density further increased to 20 mA cm⁻², the solution pH was only increased to 7.0. The extent of the increase in phosphorus and iron recovery efficiencies was only 13% and 16%, respectively, but the energy consumption increased nearly two fold to 26.66 kWh (kg P)⁻¹.

Furthermore, the effect of current density on the phosphorus recovery performance in the coupled process was also evaluated in terms of the current efficiency. Figure 5b shows that the current efficiency reached 79%, 75%, 54% and 43% at a current density of 5, 10, 15 and 20 mA cm⁻², respectively. According to the analysis of the Nernst–Planck equation, the reason for this decreasing trend is that the potential increases with increasing current density, which in turn promotes the electromigration and neutralization reactions of H⁺ and OH⁻ into the bulk solution, limiting the increase in the concentration of H⁺ and OH⁻ in the electrolyzed effluent [27]. In view of the optimal pH for vivianite formation, which ranges from 6.0 to 9.0 [28], and the combination of phosphorus and iron recovery, current efficiency and energy consumption for phosphorus recovery, a current density of 10 mA cm⁻² was selected for phosphorus resource recovery in the coupled electrolysis–microfiltration system in this study.

3.3. Effect of Coexisting Ions (HCO₃⁻ and SiO₄²⁻)

In view of the complexity of wastewater matrices, bicarbonate (HCO₃⁻) and silicate (SiO₄²⁻) probably impact the phosphorus recovery, owing to their buffering effects [29]. Thus, to show the feasibility of the coupled system for phosphorus recovery from wastewater with complex water matrices, we investigated the effects of HCO₃⁻, and SiO₄²⁻ contents on the phosphorus recovery in the range of 0–2 mM. As shown in Figure 6, the presence of HCO₃⁻ with the concentration of 0.5~2 mM had an evident effect on the phosphorus recovery performance. The pH of the alkaline solution produced from the electrolytic reactor could reach 11.5 when HCO₃⁻ was not present in the solution, and the pH was reduced to 6.6 after mixing with the acid pickling wastewater, and 80% phosphorus recovery was achieved with energy consumption of 9.15 kWh (kg P)⁻¹. But, when 0.5 mM HCO₃⁻ was present in the solution, the pH of the mixed solution was reduced, and the phosphorus recovery efficiency decreased to 74%, and the energy consumption was elevated to 10.41 kWh (kg P)⁻¹. A further increase in the HCO₃⁻ concentration to 2 mM reduced the phosphorus recovery efficiency to 57% with the mixed solution pH decreasing below 6.0, accompanying energy consumption increasing to 13.5 kWh (kg P)⁻¹. As shown in Figure 6b, the removal of HCO₃⁻ was 40%, 25% and 20%, corresponding to a reduction in concentration of 0.2 mM, 0.25 mM and 0.4 mM, respectively. In the alkaline effluent of the electrolyzer, HCO₃⁻ bound with OH⁻ to lower the pH of the mixed solution. It is well known that the solution pH affects the composition of the precipitated phase, which was influenced by the supersaturation of the precipitated phase [30]. Therefore, a decrease in the alkaline effluent pH from the electrolysis reactor would have resulted in a decrease in the saturation of vivianite compounds (Figure 2b). On the other hand, HCO₃⁻ reacted with Fe²⁺ to form precipitates or complexes, therefore reducing the concentration of free iron ions in the water, which in turn affected the vivianite formation.

SiO₄²⁻ has many functions in water treatment such as good flocculation, corrosion inhibition, antibacterial properties, pH adjustment and the removal of heavy metal ions, but the presence of SiO₄²⁻ also affected the solubility and precipitation behavior of vivianite [31]. Fe²⁺ and Fe³⁺ in solution easily combined with SiO₄²⁻ to form stable complexes, as shown in Equations (5) and (6); SiO₄²⁻ combined with iron ions through hydroxyl groups to form an envelope structure. This complexation affected the activity and solubility of iron ions and affected the vivianite formation.

$${\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{S}\mathrm{i}\left(\mathrm{O}\mathrm{H}\right)}_4\rightleftharpoons {{\mathrm{F}\mathrm{e}\left(\mathrm{O}\mathrm{H}\right)}_2-\mathrm{S}\mathrm{i}\left(\mathrm{O}\mathrm{H}\right)}_4$$

(5)

$${\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{S}\mathrm{i}\left(\mathrm{O}\mathrm{H}\right)}_4\rightleftharpoons {{\mathrm{F}\mathrm{e}\left(\mathrm{O}\mathrm{H}\right)}_3-\mathrm{S}\mathrm{i}\left(\mathrm{O}\mathrm{H}\right)}_4$$

(6)

As shown in Figure 6c, the pH of the alkaline effluent from the electrolyzer decreased from 11.5 in the absence of SiO₄²⁻ to 11.2, 11.1 and 10.8, and from 6.5 to unfavorable levels in the range of 5.6–6.0 for the generation of vivianite after mixing with acid pickling wastewater. A decrease in pH adversely affected the recovery of phosphorus and iron. As shown in Figure 6d, the coupled electrolysis–microfiltration system achieved phosphorus recovery efficiency of 80% and iron recovery efficiency of 35% with energy consumption as low as 9.15 kWh (kg P)⁻¹ in the absence of interfering ions. As the content of silicate in the solution increased, the phosphorus recovery efficiency was gradually decreased; it decreased down to 51% at silicate content of 2 mM. However, the iron recovery efficiency gradually increased from 35% to 40% during this process as silicate content increased from 0 to 2 mM and the energy consumption increased to 12.32 kWh (kg P)⁻¹. The increase in iron recovery efficiency may have been due to the reaction of silicate with iron, which could be verified by the silicon removal efficiency of 46% to 32% in Figure 6d.

3.4. The Role of Aeration Unit

In order to reduce the phosphorus content in the final effluent below the discharge standard of electroplating industry wastewater, the filtrate with a nearly neural pH discharged from the microfiltration unit was introduced into the aeration unit [32]. In the aeration unit, sufficient oxygen was provided to oxidize Fe²⁺ into Fe³⁺, which very instantly hydrolyzed to ferric hydroxide and polymeric iron hydroxides [33]. These polymers have a large specific surface area and high positive charge and can quickly promote the flocculation and precipitation of phosphate ions in wastewater through adsorption, net trapping and other effects. The changes in solution pH and P and Fe concentration under different aeration rates are shown in Figure 7. The results show that when the pH of the effluent discharged from the microfiltration unit was 6.5, the phosphorus concentration was 18.2 mg L⁻¹. And the pH of the wastewater gradually decreased during the aeration procedure at the rates of 20, 40 and 80 mL min⁻¹. Specifically, after 30 min of aeration, the pH values decreased to 5.1, 4.7 and 4.3, respectively (Figure 7a). At the same time, the phosphorus concentration reduced evidently and could reach 7.2, 0.7 and 0.5 mg L⁻¹ (Figure 7b), respectively. In fact, the residual phosphorus concentration reached 0.9 mg L⁻¹ after only 20 min of aeration when the aeration rate was 80 mL min⁻¹, which was lower than the requirement of the discharge standard. The reason for this phenomenon is that increasing the aeration rate and extending the aeration time could promote the oxidation of Fe²⁺ into Fe³⁺, the formation of ferric hydroxide and polymeric iron hydroxides and the trapping of free phosphorus ions [34]. Therefore, to further reduce the phosphorus content in the effluent from the microfiltration unit below the discharge standard, it is feasible to extend the aeration time at a relatively low aeration rate or to shorten the aeration time at a relatively high aeration rate.

3.5. Analytical Characterization of Phosphorus-Containing Sediments

The microfiltration membrane surface precipitates were analyzed using XRD and SEM. Figure 8a shows that the XRD pattern of the sediment shows a series of diffraction peaks at 8.748°, 10.289°, 13.184°, 18.239° and 29.756°, which were produced under the experimental conditions of an initial solution pH of 8.0, a phosphorus concentration of 3 mM, a Fe²⁺ concentration of 4.5 mM and a current density of 10 mA cm⁻² [35]. This suggests that the recovered precipitates were polycrystalline compounds dominated by vivianite. SEM provided more detailed information about the crystalline structure of the precipitates [36]. As shown in Figure 8b, the precipitates were irregular particles with translucent floral and spherical crystal morphology, further indicating the formation of different phases or polycrystalline compounds of iron phosphate. The above analysis proves that the coupled electrolysis–microfiltration system successfully recovered phosphorus resources from the wastewater in the form of vivianite.

This study focused on the use of the waste acid pickling wastewater as the Fe²⁺ resource for the formation of vivianite with the in situ regulation of solution pH by a novel undivided electrolysis reactor with anode diffusion-layer separation. Thus, the developed coupled processes could recover phosphorus from wastewater by realizing the goal of “treating waste with waste”. The electrocoagulation process is another viable strategy to recover phosphorus from the wastewater and shows the comparable energy utilization for phosphorus recovery with the studied coupled processes. For example, the electrocoagulation process facilitated phosphorus recovery with efficiency higher than 95% at a current density ranging from 2 A/m² to 8 A/m², with corresponding energy consumption of 6.03~29.98 kWh/kgP [37]. However, the electrocoagulation method suffers from poor long-term operational stability and high operating costs, owing to the continual consumption of iron anode.

4. Conclusions

In this study, a high-efficiency and low-energy-consumption coupled electrolysis–microfiltration system was developed for phosphorus recovery from wastewater. The results show that the system achieved phosphorus recovery efficiency of 42~80% at an initial pH = 8, an initial phosphorus concentration of 3 mM and an iron-to-phosphorus molar ratio of 1.5 when a current density of 5~20 mA cm⁻² was applied. The corresponding energy consumption was 5.78~9.15 kWh (kg P)⁻¹ with a current efficiency of 79~43%. The presence of coexisting ions (HCO₃⁻ and SiO₄²⁻) inhibited the phosphorus recovery performance. After treatment, the phosphorus concentration in the final effluent was reduced to 0.5 mg L⁻¹, below the discharge standard. Overall, this study provides a green strategy for the recovery of iron phosphate from electroplating industrial wastewater and mitigates the phosphorus shortage and potential phosphorus contamination.