Electrochemical phosphate removal and recovery unit

Abstract

Electrochemical systems and methods for removing and recovering phosphorus in the form of phosphate from aqueous solutions are provided. Phosphate radical removal is performed in an electrochemical cell having an electrode containing bismuth (Bi), zinc (Zn), copper (Cu), iron (Fe), or an oxide thereof. During the removal of phosphate radicals from an aqueous solution, the metal (Bi, Zn, Cu or Fe) and / or metal oxide of the electrode is converted into its corresponding metal phosphate within the electrode by a reversible conversion reaction. The phosphate stored in the electrode during the removal step is subsequently released into the recovery solution by electrochemical reduction of the metal phosphate to a metal.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims the priority of U.S. Non - Provisional Patent Application No. 18 / 066,474, filed on December 15, 2022, the entire content of which is incorporated herein by reference.

[0003] Reference to Government Rights

[0004] This invention was made with government support under Grant No. 1803496 awarded by the National Science Foundation. The government has certain rights in this invention. Background of the Invention

[0005] With the continuous growth of the population, a corresponding growth in the agricultural industry is needed to feed the world. Phosphorus (P) is one of the main components of fertilizers and is also a key element used in various industrial manufacturing processes. Therefore, the global phosphate rock reserves (from which P is mainly obtained) will be depleted within ~40 to 400 years, which is a major problem. (A.R. Jupp et al., Chem. Soc. Rev. 50, 87 - 101 (2021); S.J. van Kauwenbergh, World phosphate rock reserves and resources. (2010). The International Fertilizer Development Center (IFDC).)

[0006] After food fertilized with phosphorus and grown with fertilizers is consumed by humans, phosphate enters natural water sources through the runoff of phosphorus - containing fertilizers and the treatment of phosphorus - containing sewage. Phosphate ions present in water cause excessive growth of a large number of algae as excessive nutrients, leading to eutrophication, which is another serious environmental problem caused by phosphate ions. Although various methods have been developed to regulate the phosphate ion level in wastewater (e.g., metal phosphate precipitation and enhanced biological phosphorus removal), currently, the selective removal of phosphate ions present in a mixture of other substances commonly present in wastewater cannot be cost - effectively recovered as common phosphoric acid - based chemicals such as H3PO4. Considering the limited P reserves and the harmful environmental impacts of P mining, H3PO4 production, and phosphate ion treatment, there is a great need to develop a cost - effective method to recycle waste phosphate as high - quality H3PO4 or phosphate ion solution to ensure the supply of P and protect the environment. Summary of the Invention

[0007] An electrochemical system and method are provided for removing and recovering phosphorus in one or more phosphate forms from an aqueous solution.

[0008] One embodiment of an electrochemical system includes a phosphate removal unit that includes: a first electrode in an aqueous solution containing phosphate, the first electrode including a metal, a metal oxide, or a combination thereof, where the metal, metal oxide, or combination thereof is selected from bismuth, bismuth oxide, zinc, zinc oxide, copper, copper oxide, iron, and iron oxide. The phosphate removal unit may optionally include an additional electrode that serves as a cathode (referred to as the cathode for phosphate removal) when the first electrode serves as an anode during phosphate removal. The electrochemical system further includes a phosphate recovery unit that includes: a phosphatated first electrode that serves as a cathode during phosphate recovery; and an additional electrode that serves as an anode (referred to as the anode for phosphate recovery).

[0009] One embodiment of a method for removing phosphorus in the form of phosphate from an aqueous solution containing the phosphate uses an electrochemical system of the type described herein and includes the steps of: at least partially converting the metal, metal oxide, or combination thereof in the first electrode to a metal phosphate phase in the first electrode, the metal phosphate phase including a metal phosphate selected from bismuth phosphate, zinc phosphate, copper phosphate, and iron phosphate; replacing the phosphate ion-containing solution with a second aqueous solution; and applying a voltage across the phosphatated first electrode and the anode for phosphate recovery, where the voltage drives the reduction of the metal phosphate phase in the first electrode, releases phosphate ions into the second solution, and drives an oxidation reaction at the anode for phosphate recovery. The conversion of the metal and / or metal oxide in the first electrode to a metal phosphate (referred to herein as phosphatation of the first electrode) can be carried out electrochemically by pairing the first electrode with the cathode for phosphate removal, or can be carried out non-electrochemically.

[0010] After reading the following drawings, detailed description, and appended claims, other principal features and advantages of the invention will become apparent to those skilled in the art. Description of the Drawings

[0011] Exemplary embodiments of the invention will be described below with reference to the drawings, in which like reference numerals represent like elements.

[0012] Figure 1 is a schematic diagram of a phosphate removal (left figure) and recovery (right figure) system.

[0013] Figures 2A - 2B shows a more detailed illustration of an electrochemical phosphate removal unit including a bismuth electrode ( Figure 2A ) and a corresponding electrochemical phosphate recovery unit for this example ( Figure 2B ).

[0014] Figure 3A Shows the linear sweep voltammetry (LSV) curve of the Bi electrode obtained by scanning the potential in the positive direction from the open circuit potential (OCP) in 1 M NaH₂PO₄. By in 1 M NaH₂PO₄ ( Figure 3B ) and in 2.5 M H₃PO₄ ( Figure 3C ) the LSV of the BiPO₄ electrode was obtained by scanning the potential in the negative direction from the OCP (scan rate, 1 mV / s). The dashed line represents the equilibrium reduction potential of BiPO₄ to Bi (V vs. Ag / AgCl) under each condition.

[0015] Figure 4A Shows the J - t plot obtained for the conversion of Bi to BiPO₄ (phosphate removal) in 1 M NaH₂PO₄ at 0.95 V (vs. Ag / AgCl). Figure 4B Shows the J - t plot obtained for the conversion of BiPO₄ to Bi (phosphate recovery) in 2.5 M H₃PO₄ at - 0.43 V (vs. Ag / AgCl). Figure 4C Shows the cyclic test of the phosphorylation of Bi in 1 M NaH₂PO₄ and the dephosphorylation of BiPO₄ in 2.5 M H₃PO₄ for 30 cycles. Detailed Description

[0016] An electrochemical system and method for removing and recovering phosphorus in one or more phosphate forms from an aqueous solution are provided. The removal of phosphate is carried out in an electrochemical cell comprising a first electrode comprising a metal, a metal oxide, or a combination thereof or consisting of a metal, a metal oxide, or a combination thereof, wherein the metal or metal oxide is selected from bismuth (Bi), bismuth oxide, zinc (Zn), zinc oxide, copper (Cu), copper oxide, iron (Fe), or iron oxide. In some embodiments of the system and method, during the process of removing phosphate from the aqueous solution, the metal (Bi, Zn, Cu, or Fe) of the electrode is oxidized in the first electrode to one or more corresponding metal phosphates (i.e., one or more bismuth phosphates, one or more zinc phosphates, one or more copper phosphates, or one or more iron phosphates). In other embodiments of the system and method, the metal oxide is converted to one or more corresponding metal phosphates by ion exchange. The exact composition of the metal phosphate will vary depending on the metal type, solution composition, and removal conditions. The phosphate stored in the first electrode during the removal step is then released into the recovery solution by the electrochemical reduction of the metal phosphate.

[0017] The electrochemical systems and methods described herein have been achieved, at least in part, based on the inventors' discovery that electrodes of metals Bi, Zn, Cu, and Fe or their oxides have the ability to store phosphate by forming metal phosphates within the electrodes and subsequently release the stored phosphate by reverting back to the metal via a reversible solid-state conversion reaction between the metal (i.e., Bi, Zn, Cu, or Fe) and its corresponding phosphate upon reduction. Notably, the metal phosphate phase forms not only on the surface but also within the bulk of the metal electrode, enabling high phosphate removal capacity.

[0018] When a voltage is applied between the first electrode (i.e., the anode in the phosphate removal step) and the cathode for phosphate removal, oxidation of the metal in the electrode can occur electrochemically during the phosphate removal step. However, in some cases, phosphate removal can also be achieved without applying a voltage. One such case is when dissolved oxygen (O2) or protons in the electrolyte chemically oxidize the metal in the first electrode, and the oxidized metal ions in the first electrode form metal phosphates, thereby removing phosphate from the solution. Alternatively, by replacing oxide ions with phosphate ions, the metal oxide in the first electrode can be converted to metal phosphate, thereby removing phosphate from the solution. This reaction is not an oxidation reaction because the conversion of metal oxide to metal phosphate does not involve oxidation of the metal. This non-electrochemical removal of phosphate is advantageous because it allows phosphate removal without energy input. If the phosphate-containing solution contains dissolved O2 from air, or the first electrode contains metal oxides formed by air oxidation of non-noble metals in the first electrode, non-electrochemical phosphate removal can occur to some extent even if not intended. Alternatively, the composition of the first electrode or the aqueous solution can be altered to intentionally promote non-electrochemical phosphate removal. For example, for Zn with an oxide that can be easily converted to zinc phosphate by non-oxidizing anion exchange in a concentrated phosphate solution, a zinc oxide electrode can be used instead of a Zn electrode because under given solution conditions, non-electrochemical phosphate removal can be as effective as electrochemical phosphate removal.

[0019] Figure 1 A schematic diagram of an electrochemical system and method for removing and recovering phosphate from an aqueous solution is shown. In this embodiment of the method, the phosphate removal step is carried out electrochemically. The system includes an electrochemical cell having a first electrode 102 and a cathode 104 for phosphate removal connected by an external circuit or wire, and a voltage source 106 configured to apply a voltage across the first electrode 102 and the cathode 104. The first electrode 102 and the cathode 104 are in contact with an aqueous solution containing phosphate ions (e.g., H2PO4 - 、HPO4 2- and / or PO4 3-) is contacted with (e.g., immersed in) the aqueous electrolyte solution 108. The first electrode 102 comprises or consists of a metal selected from Bi, Zn, Cu, and Fe (generally denoted as "metal" in Figure 1 ). During the electrochemical phosphate removal step ( Figure 1 , left figure), a voltage is applied across the metal electrode 102 and the cathode 104, and the metal of the first electrode is oxidized to form one or more metal phosphates (generally denoted as "metal phosphates" in Figure 1 ), resulting in the removal of phosphate from the solution 108, and an electrochemical reduction reaction occurs at the cathode 104. (The external voltage for oxidizing the metal applied across the first electrode 102 and the cathode 104 for phosphate removal is herein referred to as the metal oxidation voltage.) In Figure 1 exemplary embodiments, the cathode 104 performs the reduction of water to H2. To facilitate the hydrogen evolution reaction, the cathode 104 can consist of a hydrogen evolution catalyst, such as platinum, ruthenium, or iridium. However, it should be understood that the water reduction is only an exemplary instance of the reduction reaction that can be performed by the cathode 104. As an alternative, other more convenient or useful reduction reactions can be carried out. By way of illustration, an O2 reduction, metal deposition, or ion storage reaction (e.g., Na + insertion into a Na-storage electrode) can be carried out at the cathode 104. The composition of the cathode 104 will vary depending on the reaction to be employed. Once the phosphate removal is complete, the solution 108 is replaced with a second aqueous solution 110 called the recovery solution. By removing the solution 108 from the electrochemical cell and replacing it with the recovery solution 110, the phosphate removal unit is thus converted into a phosphate recovery unit, or by removing the phosphatized first electrode 102 from the solution 108 and placing it in another electrochemical cell containing the recovery solution 110, the initial phosphate-containing aqueous solution 108 can be replaced with the recovery solution 110.

[0020] In embodiments of the method in which the phosphatization of the first electrode and the phosphate removal are carried out completely non-electrochemically, the cathode 104 can be omitted. Additionally, in embodiments of the method using an electrode containing a metal oxide, the first electrode 102 will comprise or consist of a metal oxide selected from bismuth oxide, zinc oxide, copper oxide, and iron oxide.

[0021] In the subsequent phosphate recovery step ( Figure 1, (right figure), a reverse voltage is applied across the first phosphorylated electrode 102 and the counter electrode, and the counter electrode serves as an anode for phosphate removal. During phosphate recovery, the metal phosphate phase formed in the first phosphorylated electrode 102 is reduced back to its metal form, and the phosphate ions previously stored in the first electrode 102 are released into the recovery solution 110. At the same time, an electrochemical oxidation reaction occurs at the anode for phosphate recovery. (The external voltage applied across the first phosphorylated electrode and the anode for phosphate recovery to reduce the metal phosphate is referred to herein as the metal phosphate reduction voltage.) If the electrochemical reduction in the phosphate removal step and the electrochemical oxidation in the phosphate recovery step can be carried out by the same electrode, the anode for phosphate recovery can be the same electrode that serves as the cathode for phosphate removal. That is, the cathode 104 for phosphate removal can be used as the anode for phosphate recovery in the subsequent phosphate recovery step. Alternatively, a different anode 112 can be used for phosphate recovery in the electrochemical phosphate recovery step. The composition of the material of the different anode 112 for phosphate recovery will depend on the reaction occurring at that anode. In Figure 1 the illustrative embodiment of x , the anode 112 for phosphate recovery is an oxygen evolution catalyst electrode, such as oxides of nickel and iron and noble metal oxides (such as RuO x ). However, it should be understood that this is only an exemplary instance of the oxidation reaction that can be carried out by the anode 104 or 112 for phosphate recovery. By way of illustration, an ion release reaction can occur at the anode for phosphate recovery, such as the deintercalation of cations (such as Na + ) from an ion storage electrode (such as a Na storage electrode). The electrodes 102, 104, and 112 can optionally further include other materials commonly found in electrodes, including polymer binders, conductive additives, and / or current collectors or other support substrates.

[0022] The electrochemical system is capable of preferentially removing phosphate ions over other anions present in the aqueous solution, and depending on the first electrode used in phosphate removal, phosphate can be recovered as a phosphoric acid solution or a phosphate solution. Therefore, the electrochemical system and method can be used to treat various waste solutions containing phosphate ions that are not currently recycled and reused to produce a sustainable phosphate cycle.

[0023] Because the electrochemical phosphate removal unit converts the metal and / or metal oxide of the electrode into a metal phosphate phase, thereby storing phosphate in the electrode, the electrochemical systems and methods described herein are readily distinguishable from electrocoagulation systems, which oxidize sacrificial metal electrodes into their corresponding soluble metal ions, and the soluble metal ions then react with phosphate ions in solution to form metal phosphates that precipitate out of the solution and are collected as sludge. (In fact, the oxidation of the metal electrode into soluble metal ions is considered an undesirable reaction for the electrochemical systems and methods described herein because it prohibits the sustainable use of the metal electrode.) The present systems and methods are also readily distinguishable from electroadsorption systems, which use charged electrodes to attract oppositely charged ions from a solution via electrostatic attraction. In electroadsorption, the oppositely charged ions are temporarily adsorbed onto the electrode surface in the presence of a bias voltage but do not form a new phase on or in the electrode and are released when the bias voltage is removed. In contrast, the system of the present invention stores phosphate through the phase change of metal to metal phosphate and / or through the phase change of metal oxide to metal phosphate, allowing phosphate to be stored not only on the surface but also in the bulk of the electrode, even when the bias voltage is removed.

[0024] The ability of the electrochemical phosphate removal method of the present invention to selectively remove and store phosphate from a solution containing a mixture of ions is also a significant feature. The selective removal of phosphate is attributed to the formation of insoluble salts of Bi, Zn, Cu, or Fe with phosphate, while common anions such as sulfate, nitrate, and chloride do not form insoluble phases with Bi, Zn, Cu, or Fe. As a result, phosphate can be selectively removed. (One possible exception is Bi, which can form insoluble BiOCl with chloride. However, when chloride and phosphate coexist, the formation of BiPO4 is highly favorable, and the formation of BiOCl is negligible at pH below 9.5.) In contrast to existing methods, other methods, such as reverse osmosis and electrodialysis, non-selectively remove all cations and anions as well as phosphate, thus requiring additional ion separation steps or rendering the recovered materials unusable and / or non-recyclable.

[0025] It should be noted that although phosphate can be the sole or main product of metal oxidation during the phosphate removal step, metal oxides can also be formed in addition to metal phosphates if there is not enough phosphate at the electrode surface to form metal phosphates. This can occur when using a diluted phosphate solution and the phosphate at the electrode surface is depleted because the rate of metal phosphate formation is faster than the mass transfer rate of phosphate from the bulk solution to the electrode surface. This can reduce the Faraday efficiency (FE) of phosphate removal. However, the co-formation of metal oxides is not particularly problematic because the metal oxides can be converted back to metal during the phosphate recovery step without introducing any impurity anions into the recovery solution. To minimize the formation of metal oxides during phosphate removal in a dilute phosphate solution, the metal oxidation voltage can be applied only for a short period of time, followed by a resting time with no voltage applied, and this sequence can be repeated without applying a continuous metal oxidation voltage. During the resting time, the phosphate consumed at the electrode surface can be replenished by the diffusion of phosphate from the bulk solution, and this can ensure more efficient phosphate removal during the next oxidation.

[0026] Another side reaction that can occur but is desirable to avoid is the continuous dissolution loss of the metal in the electrode. For example, the oxidative dissolution of Zn, Cu, and Fe can occur in acidic solutions (e.g., pH < 2). Therefore, the solution pH for the phosphate removal and recovery unit should be high enough (e.g., pH > 4) to prevent the dissolution loss of the metal in the electrode.

[0027] The unique advantage of Bi and BiPO4 is that they are insoluble and stable in concentrated H3PO4 solutions with a pH as low as 0.25. Therefore, when a BiPO4 electrode is used for the electrochemical recovery of phosphate and the reaction of reducing BiPO4 to Bi is combined with the water oxidation reaction (producing H + ) at the anode for phosphate recovery, phosphate can be recovered as phosphoric acid (H3PO4(aq)) in an acidic recovery solution (i.e., a recovery solution with pH < 7.0, including recovery solutions with pH < 6, < 5, < 4, and < 3) without dissolving Bi or BiPO4.

[0028] Different from Bi and BiPO4, other metals (Zn, Cu, and Fe) and their corresponding metal phosphates are unstable in strongly acidic solutions. Therefore, a slightly acidic, neutral, or basic phosphate solution should be used for the recovery step, and phosphate will be recovered as a highly pure concentrated phosphate solution at a slightly acidic, neutral, or basic pH. For illustrative purposes only, the pH of the recovery solution can be in the range of 4 to 14. However, pH values outside these ranges can be used.

[0029] Electrochemical systems and methods can be used to remove phosphate from a variety of aqueous solutions, including residential, municipal, and / or industrial wastewater treatment processes and facilities. To make the electrochemical removal more effective, optionally, the phosphate in the water can be concentrated prior to phosphate removal. In some embodiments of the method, the electrochemical phosphate removal and recovery system of the present invention is located downstream of another water treatment system that concentrates the phosphate in its effluent or other output. For example, the electrochemical phosphate removal and recovery system described herein can be used downstream of other currently used phosphate removal processes (e.g., chemical precipitation, chemical and electrocoagulation, enhanced biological phosphate removal) that produce phosphate-containing sludge from dilute phosphate-containing wastewater. These phosphate-containing sludges can be collected and dissolved to produce a concentrated phosphate solution, and using the electrochemical systems and methods described herein, phosphate can be selectively removed from the concentrated phosphate solution and recovered as high-purity H3PO4 or other high-purity phosphate solutions. This is important because the phosphate removed by these other methods cannot currently be used to produce high-quality phosphate chemicals because the metal phosphate precipitates contain various impurities that are difficult to separate in a cost-effective manner.

[0030] If the concentrated phosphate solution for phosphate removal is prepared by redissolving metal phosphate precipitates (e.g., iron phosphate) obtained by chemical or electrochemical coagulation methods, the reduction reaction at the cathode for phosphate removal in the phosphate removal unit can be metal deposition (e.g., Fe deposition), which recovers the metal ions used in the upstream chemical precipitation or electrocoagulation step, making the entire phosphate removal and recovery process even more sustainable.

[0031] By way of illustration, the phosphate-containing aqueous solutions described herein can have an initial phosphate concentration of at least 10 -6 M, at least 10 -3 M or at least 0.1 M. For example, phosphate-containing aqueous solutions having an initial phosphate concentration in the range of 0.1 to 1 M can be used. However, phosphate-containing aqueous solutions having higher or lower initial phosphate concentrations can also be used. As used herein, the phrase "initial phosphate concentration" refers to the concentration of phosphate in the solution before the start of the phosphate removal step.

[0032] Examples

[0033] Example 1: Removal of phosphate and recovery using a bismuth electrode.

[0034] This example illustrates a phosphate removal and recovery system using a Bi electrode. In the phosphate removal unit ( Figure 2A ), the Bi electrode is used as the anode and is oxidized to BiPO4, storing phosphate (Equation 1). (H2PO4 is used in Equation 1 -The anion, as it is the major species present under the pH conditions studied in this example, but the exact phosphate species can vary with solution pH.) This reaction is combined with the cathodic reaction for the reduction of water to H2 (Equation 2-3).

[0035] Phosphate removal unit

[0036] Anodic reaction: Bi(s) + H2PO4 - (aq) → BiPO4(s) + 2H + + 3e - (Equation 1)

[0037] Cathodic reaction: 3H + + 3e - → 3 / 2H2(g) (Equation 2)

[0038] Overall reaction: Bi(s) + H2PO4 - (aq) + H + → BiPO4(s) + 3 / 2H2(g) (Equation 3)

[0039] When the conversion of Bi to BiPO4 is complete, the resulting BiPO4 electrode will be used as the cathode in the phosphate recovery unit ( Figure 2B ), where it is reduced back to Bi, releasing phosphate into the solution (Reaction Equation 4). The anode in this unit oxidizes water to O2, generating H + (Equation 5). As a result, H3PO4 is produced and accumulates in the recovery unit without the need to add acid (Reaction Equation 6).

[0040] Phosphate recovery unit

[0041] Cathodic reaction: BiPO4 + 3H + + 3e - → Bi + H3PO4(aq) (Equation 4)

[0042] Anodic reaction: 3 / 2H2O → 3H + (aq) + 3e - + 3 / 4O2 (Equation 5)

[0043] Overall reaction: BiPO4(s) + 3 / 2H2O → Bi(s) + H3PO4(aq) + 3 / 4O2 (Equation 6)

[0044] Experiments have confirmed that the conversion reaction between Bi and BiPO4 can occur without causing the oxidation or reduction of water. Figure 3AThe LSV with a Bi electrode is shown, where the potential is swept in the positive direction from the OCP in a 1 M NaH₂PO₄ solution (pH 3.98). It shows a sharp anodic peak centered at approximately -0.05 V and a broad anodic peak centered at approximately 0.8 V, both due to the formation of BiPO₄. This assumption is supported by the results of the potentiostatic phosphatization of Bi and the crystal structure of BiPO₄ discussed below. After these two anodic peaks, no other anodic features attributed to the oxygen evolution reaction (OER) were observed, meaning that the oxidation of water is inhibited on the surface of BiPO₄. This is highly beneficial as it means that the OER cannot reduce the FE of the phosphate storage reaction of Bi.

[0045] The LSV of the BiPO₄ electrode was also recorded, where the potential was swept in the negative direction from the OCP ( Figure 3B ). It has two reduction peaks around -0.58 V and -0.75 V, both of which are due to the reduction of BiPO₄ to Bi. The reduction wave for water reduction appears at a more negative potential than these reduction peaks, meaning that a potential can be selected to reduce BiPO₄ to Bi without causing water reduction.

[0046] The LSV of the BiPO₄ electrode in 2.5 M H₃PO₄ (pH = 0.25) was also recorded to simulate the electrolyte of the phosphate recovery unit, in which the phosphate released from the BiPO₄ electrode will accumulate as a concentrated H₃PO₄ solution ( Figure 3C ). In this solution, the reduction of water also occurs at a more negative potential than the reduction of BiPO₄ to Bi, meaning that the reduction of water does not interfere with the phosphate release reaction of BiPO₄, even in this strongly acidic solution. In fact, the onset of the BiPO₄ reduction peak in 2.5 M H₃PO₄ is closer to the thermodynamic potential of the Bi / BiPO₄ couple than in 1 M NaH₂PO₄, meaning that the reduction of BiPO₄ in 2.5 M H₃PO₄ has better kinetics than in 1 M NaH₂PO₄.

[0047] Next, the potentiostatic conversion rate of Bi to BiPO₄ was detected in 1 M NaH₂PO₄ at 0.95 V (versus Ag / AgCl) ( Figure 4A), to quantitatively evaluate how much Bi can be converted to BiPO4 and FE for the phosphate removal reaction. For this and the remaining electrode performance tests, sheet-type Bi electrodes were prepared, in which Bi particles were mixed with a conductive carbon additive and a polytetrafluoroethylene (PTFE) binder. This type of fabrication is commonly used to produce electrodes for battery applications because it enhances electron and ion conduction throughout the electrode. This fabrication method also alleviates any comminution-related problems of the electrode that can be caused by volume changes during the phosphate storage and release reactions of Bi (i.e., a 259% volume expansion when Bi is converted to BiPO4).

[0048] The area under the J-t curve is equal to the total charge passed during the conversion of Bi to BiPO4 (7.16 C). This charge can be related to the amount of Bi converted to BiPO4 during oxidation and compared to the amount of Bi present in the electrode; it was calculated that approximately 56% of the Bi in the Bi electrode was converted to BiPO4 under the given oxidation conditions. This result is notable because a conversion rate of ~56% means that BiPO4 is formed not only as a surface passivation layer but also through the conversion of Bi in the bulk.

[0049] The X-ray diffraction (XRD) pattern of the Bi electrode before the potentiostatic phosphatization of Bi showed peaks corresponding to rhombohedral Bi (space group R-3m). (Graphite and PTFE binder peaks were also present.) After oxidation, the intensity of the Bi diffraction peaks in the XRD pattern decreased, and new peaks appeared. These new peaks corresponded to trigonal BiPO4·0.67H2O (space group P3121), confirming that the Bi electrode did store phosphate through a solid-state conversion reaction. BiPO4·0.67H2O has large channels along the c-axis that contain water molecules. (B. Romero et al., Inorg. Chem. 33, 1869 - 1874 (1994).) Even after the Bi surface was passivated by the BiPO4 layer, the size of the channels and the flexible configuration of the water molecules allowed phosphate ions and water molecules to diffuse through these channels from the solution and continuously convert the underlying bulk Bi to BiPO4, thus allowing bulk conversion. At a conversion rate of 56% of Bi to BiPO4, 1 g of Bi can remove 0.254 g of phosphate.

[0050] The faradaic efficiency (FE) of the phosphate removal reaction of the Bi electrode was detected by comparing the charge passed through the Bi electrode during oxidation and the actual change in the phosphate concentration of the electrolyte. The results showed an FE of ~108% (Table 1). Such a high FE is very consistent with the LSV results, indicating the absence of OER. The phosphate removal efficiency was slightly higher than 100%, presumably because Bi was chemically oxidized by O2 to BiPO4, which allowed the removal of phosphate without consuming charge.

[0051] Table 1. Faradaic efficiency for phosphate removal in solutions with various initial phosphate concentrations.

[0052]

[0053] Solutions containing different concentrations of phosphate were used to study how phosphate concentration affects the FE for phosphate removal. The results showed that the FE for phosphate removal decreased with decreasing phosphate concentration, calculated as 98% and 58% at phosphate concentrations of 500 mM and 100 mM, respectively. The decrease in FE with decreasing phosphate concentration was attributed to the oxidation of Bi to Bi2O3 rather than BiPO4 when phosphate was insufficient at the interface. This result indicates that the phosphate removal unit will operate more efficiently when the phosphate concentration is high. However, although the formation of Bi2O3 during the phosphorylation of Bi in dilute phosphate solutions reduces the FE for phosphate removal, the presence of Bi2O3 in the BiPO4 electrode does not pose any serious problems for the recovery of phosphate in the form of H3PO4 in the phosphate recovery unit, except for reducing the FE for the phosphate recovery reaction. Therefore, although the FE is affected by the phosphate concentration, the Bi / BiPO4 system can recycle phosphate into H3PO4 in a unique and robust way. (To increase the FE for phosphate removal in low-concentration phosphate solutions, the above method of alternating short oxidation steps and resting steps can be applied)

[0054] The potentiostatic reduction of BiPO4 to Bi was also tested in 2.5 M H3PO4 at -0.43 V (versus Ag / AgCl), and the resulting J-t curve is shown in Figure 4B . This electrolyte condition was chosen to simulate the electrolyte of the phosphate recovery unit after H3PO4 had accumulated to form a concentrated H3PO4 solution. The charge passed during dephosphorylation was calculated to be 7.48 C, which is comparable to the charge (7.16 C) used to oxidize Bi to BiPO4. This result implies that BiPO4 can be reversibly and completely reduced back to Bi without any loss of BiPO4 or Bi in this highly acidic and concentrated phosphate solution. Since the FE for phosphate removal during phosphorylation was slightly higher than 100%, the dephosphorylation ability was slightly higher than the phosphorylation ability; during the reduction process, BiPO4 formed by the oxidation of Bi by O2 was also dephosphorylated without consuming charge, increasing the observed dephosphorylation charge. XRD of the BiPO4 electrode after the reduction reaction showed that the BiPO4 peak completely disappeared while the Bi peak completely recovered.

[0055] In the scanning electron microscope (SEM) images of the Bi electrode after phosphorylation and dephosphorylation, no obvious changes were shown on the surface after phosphorylation and dephosphorylation. This confirmed that the conversion between Bi and BiPO4 did not cause dissolution and precipitation of the electrode material, and the sheet electrode in which each Bi particle was well mixed with carbon and binder could uniformly perform the Bi / BiPO4 conversion. The conversion between Bi and BiPO4 was repeated 30 cycles ([ Figure 4C ) using the above constant potential phosphorylation and dephosphorylation conditions. After a slight initial change, the charge passed during phosphorylation and dephosphorylation was constant, indicating that the Bi / BiPO4 electrode could be continuously used for phosphate removal and phosphoric acid recovery as H3PO4.

[0056] Method

[0057] Materials. Bi2O3 (99.999%, PURATREM), graphite (99.995%, Sigma-Aldrich), PTFE (60 wt% dispersion in H2O, Sigma-Aldrich), colloidal graphite (isopropyl alcohol, Ted Pella, Inc.), BiCl3 (≥98%, Sigma Aldrich), polyethylene glycol (PEG) (molecular weight 6000, USB Corporation), HCl (Sigma-Aldrich, 37%), NaCl (99%, Macron), HNO3 (70%, Sigma Aldrich), NaH2PO4 (≥99%, Sigma Aldrich) and H3PO4 (≥85%, Sigma-Aldrich) were used without further purification. Deionized water (Barnstead E-pure water purification system, resistivity >18 MΩ·cm) was used to prepare all solutions.

[0058] Preparation of Bi electrode. The Bi electrode for LSV experiments was prepared by electrodeposition according to the procedure reported in previous studies. (D-H Nam et al., J. Am. Chem. Soc. 139, 11055–11063 (2017).) An undivided three-electrode cell was used, in which a titanium (Ti) sheet was used as the working electrode, a platinum (Pt) sheet was used as the counter electrode, and a saturated calomel (SCE) electrode was used as the reference electrode. The Ti sheet was masked to expose an area of 1 cm 2 . An aqueous solution containing 14 mM BiCl3, 1.4 M HCl and 2.5 g / L PEG 6000 was used as the plating solution. Constant potential deposition was carried out by applying a potential of -2.6 V (versus SCE) for 2 minutes, resulting in the deposition of Bi (Bi 3+ +3e - →Bi, E o= 0.286 V (vs. SHE). During deposition, the solution was stirred at 300 rpm and the average deposition current was ~900 mA / cm 2 . After deposition, the Bi electrode was rinsed with water and dried in air. The obtained Bi electrode had a high surface area macroporous foam structure. (D-H. Nam et al., 2017.)

[0059] For all other electrochemical experiments, a pelletized Bi electrode was prepared by a ball milling process followed by a rolling process. (D-H. Nam et al., Energy Storage Mater. 37, 556 556-566 (2021).) First, Bi2O3 and graphite powder (mass ratio of active material to graphite powder was 2.5:1) were mixed with a mortar and pestle, and then the mixture was ball milled at a rate of 1060 cpm for 1 h using a high-energy ball mill (8000M mixer / mill, from SPEX SamplePrep). The obtained composite was mixed with a PTFE binder (mass ratio of 10:6) using water as a solvent to form a thick slurry. The slurry was kneaded repeatedly in a mortar and pestle, and then rolled and pressed into a thin electrode pellet with a thickness of ~100 μm. Finally, the electrode pellet was dried on a hot plate at 80 °C for at least 6 h to remove water and residual organic compounds. The dried electrode pellet was cut into 1 cm 2 electrodes, which were then attached to a graphite current collector with colloidal graphite paste for electrochemical testing. Bi2O3 powder was used instead of Bi powder for this method because it was found that using Bi2O3 powder resulted in a higher quality pelletized electrode. The obtained Bi2O3 electrode was converted into a Bi electrode by the following activation / reduction method and then used for phosphate removal. The Bi2O3 electrode was immersed in 0.6 M NaCl and alternately reduced and oxidized at a current density of ±3 mA cm -2 for about 20 cycles with cut-off potentials of -1.3 V and 0.8 V (vs. Ag / AgCl), respectively. The process ended with a reduction cycle to form the Bi electrode. This cycling process ensured that the pelletized electrode was properly wetted and that the Bi particles in the electrode were in maximum contact with the electrolyte. The mass of Bi in the electrode was 9.23 mg / cm 2 .

[0060] Characterization. A LEO Supra55 VP scanning electron microscope (SEM) and a powder X-ray diffractometer (XRD) (Bruker D8 Advanced PXRD, Ni-filtered Cu Kα radiation, at an accelerating voltage of 2 kV were used )The morphology and crystal structure of the Bi electrode were detected separately. Energy-dispersive X-ray spectroscopy (EDS) was performed at an acceleration voltage of 12 kV using the same SEM equipped with EDS (Noran System Seven, Thermo Fisher). To study the phase transformation of the Bi electrode after oxidation (phosphatization) and reduction (dephosphatization), ex-situ SEM and XRD analyses were carried out on the cycled samples. The Faradaic efficiency of the Bi electrode for phosphate removal was calculated by quantifying the amount of P present in the solution before and after the phosphate removal process using inductively coupled plasma optical emission spectroscopy (ICP-OES) (Agilent 5110).

[0061] Electrochemical experiments. LSV and potentiostatic phosphatization / dephosphatization experiments were carried out in an undivided three-electrode cell. The Bi electrode was used as the working electrode, with a Pt counter electrode and a double-junction Ag / AgCl (4M KCl) reference electrode. The Pt electrode was prepared by sputter coating a 100 nm thick Pt layer on a clean glass slide (LGA film) with a 20 nm thick Ti adhesion layer. Using the same Bi electrode, the reusability of the Bi electrode was studied by repeatedly performing the phosphatization process in 1M NaH2PO4 (pH 3.98) and the dephosphatization process in 2.5M H3PO4 (pH 0.25). The phosphatization and dephosphatization processes were carried out potentiostatically at 0.95 V (vs. Ag / AgCl) and -0.43 V (vs. Ag / AgCl) for 10 minutes and 15 minutes, respectively. After each process was completed, the Bi electrode was manually lifted from the solution, rinsed with DI water, and transferred to another solution for the next process.

[0062] Calculation of the equilibrium redox potential of Bi / BiPO4.

[0063] 1) Equilibrium potential in 1M NaH2PO4 (pH 3.98)

[0064]

[0065] 2) Equilibrium potential in 2.5M H3PO4 (pH 0.25)

[0066]

[0067] The theoretical charge required for phosphate removal by Bi.

[0068] When it is assumed that Bi is completely converted to BiPO4 through the following reaction,

[0069] Bi(s)+H2PO4 - (aq)→BiPO4(s)+2H + +3e -

[0070] The total charge required to completely convert Bi to BiPO4 is calculated as follows,

[0071]

[0072] where Q is the charge in coulombs, F is the Faraday constant (96485.33 C / mol), n is the number of electrons involved in the reaction, and M is the molar mass of Bi. The Bi electrode used in this study contained 9.23 mg of Bi. Therefore, 12.78 C was required for the complete phosphatization of the Bi electrode. However, during the phosphatization of Bi, the charge observed experimentally was 7.16 C, corresponding to 56% of the expected total charge. This means that under the oxidation conditions used in this study, approximately 56% of the Bi in the Bi electrode was converted to BiPO4.

[0073] Phosphate removal capacity of the Bi electrode.

[0074] Each mole of Bi (molar mass = 208.9804 g / mol) can store one mole of phosphate (molar mass = 94.97 g / mol), meaning that the theoretical capacity of Bi to remove phosphate per gram is 0.454 g / g. Bi . In our system, the conversion rate of Bi to BiPO4 was 56%, and 1 g of Bi could remove 0.254 g of phosphate.

[0075] Example 2: Removal of phosphate and recovery using an iron electrode.

[0076] This example illustrates a phosphate removal and recovery system using an Fe electrode.

[0077] A sheet-type Fe electrode was prepared as follows: Iron nanoparticles, carbon black, and PTFE were mixed in a ratio of 100:1:33 using water as a solvent to form a thick slurry. The resulting slurry was processed into an Fe sheet electrode using the steps described in Example 1 for preparing the Bi sheet electrode.

[0078] Phosphate was removed from a 0.1 M NaH2PO4 (pH 7 adjusted with NaOH) solution by immersing the Fe electrode in the solution. Before the phosphate removal step, a constant potential of -1.0 V (versus Ag / AgCl) was applied to the Fe electrode for 5 minutes to reduce any iron oxides present on the electrode surface. After this reduction process, a potential of 1.2 V (versus Ag / AgCl) was applied for 5 s for phosphate removal, followed by a rest time of 45 s without applying a potential. This rest time was used to replenish the phosphate at the electrode surface to ensure that more phosphate could be incorporated into the Fe electrode during the oxidation process. The process of applying 5 s oxidation voltage pulses followed by 45 s rest times was repeated until the total cumulative oxidation time reached ~13 minutes.

[0079] The oxidized electrode was analyzed by XRD. Different from BiPO4, the iron phosphate formed by the phosphate removal reaction is amorphous and shows no Bragg peaks. Therefore, the chemical formula and structure of the iron phosphate formed by the phosphate removal step cannot be identified by XRD. However, when analyzing the elemental composition of the obtained electrode by EDS, the atomic ratio of Fe:P is 1:~0.2, confirming that phosphate is indeed removed from the solution and incorporated into the Fe electrode to form iron phosphate. Since not all of the Fe present in the Fe electrode (i.e., especially the core part of each Fe particle) is converted to iron phosphate within this short experimental time, the Fe:P ratio obtained by EDS cannot be used to determine the chemical formula of the iron phosphate formed in the Fe electrode. (The amount of Fe detected by EDS is the sum of the Fe in the iron phosphate and the Fe in the unreacted Fe metal). The possible iron phosphate phases formed include FePO4 and Fe3(PO4)2 (and / or their hydrated phases).

[0080] For the phosphate recovery step, the iron phosphate electrode obtained from the phosphate removal step was immersed in a 0.01 M NaH2PO4 (pH 7 adjusted with NaOH) recovery solution and reduced to Fe at a constant current of -15 mA / cm 2 for 20 minutes. The obtained electrode was analyzed by EDS, and the results showed that >90% of the phosphate stored in the electrode was removed from the electrode, meaning it was recovered in the recovery solution.

[0081] Example 3: Removal of phosphate and recovery using a copper electrode.

[0082] This example illustrates a phosphate removal and recovery system using a Cu electrode.

[0083] The sheet-like Cu electrode was prepared as follows: Using water as a solvent, copper nanoparticles, carbon black, and PTFE were mixed in a ratio of 3:1:1 to form a thick slurry. The obtained slurry was processed into a Cu sheet electrode using the procedure described in Example 1 for preparing the Bi sheet electrode.

[0084] Phosphate was removed from a 0.1 M NaH2PO4 solution (pH 7 adjusted with NaOH) by immersing a Cu electrode in the solution. Prior to the phosphate removal step, a constant potential of -0.6 V (versus Ag / AgCl) was applied to the Cu electrode for 5 minutes to reduce any copper oxides present on the electrode surface. After this reduction process, a potential of 1.2 V (versus Ag / AgCl) was applied for 5 s for phosphate removal, followed by a standing time of 45 s. This standing time was to replenish phosphate at the electrode surface to ensure that more phosphate could be incorporated into the Cu electrode upon oxidation. The process of applying a 5 s oxidation voltage pulse followed by a waiting time of 45 s was repeated until the total cumulative oxidation time reached ~13 minutes.

[0085] The oxidized electrode was analyzed by XRD. The newly formed phase appeared to be amorphous, but some crystalline regions showed XRD peaks that matched well with the peaks expected for Cu3(PO4)2·3H2O. Thus, Cu3(PO4)2·3H2O is one of the phases formed, and other amorphous copper phosphates may also form. EDS analysis showed that the Cu:P ratio was 1:~0.36, confirming that phosphate was indeed removed from the solution and incorporated into the Cu electrode. Also, since not all of the copper present in the copper electrode was converted to copper phosphate during this short experimental time, the Cu:P ratio obtained by EDS cannot be used to determine the chemical formula of the copper phosphate formed in the Cu electrode.

[0086] For the phosphate recovery step, the copper phosphate electrode obtained from the phosphate removal step was immersed in a 0.01 M NaH2PO4 recovery solution (pH 7 adjusted with NaOH) and reduced back to Cu at a constant current of -3 mA / cm 2 for 20 minutes. The resulting electrode was analyzed by EDS, and >90% of the phosphate stored in the electrode was removed from the electrode, meaning it was recovered into the recovery solution.

[0087] Example 4: Removal and recovery of phosphate using a zinc or zinc oxide electrode.

[0088] This example illustrates a phosphate removal and recovery system using a Zn or ZnO electrode.

[0089] First, a sheet-like ZnO electrode was prepared as follows: Using water as a solvent, zinc oxide nanoparticles, carbon black, and PTFE were mixed in a ratio of 3:1:2 to form a thick slurry. The resulting slurry was processed into a ZnO sheet electrode using the procedure described in Example 1 for preparing a Bi sheet electrode. Prior to the phosphate removal step, the resulting ZnO sheet electrode was electrochemically reduced to a Zn sheet electrode at a constant current of -16 mA / cm 2 in 0.01 M NaH2PO4 (pH 7 adjusted with NaOH) for 1 hour.

[0090] By immersing a Zn electrode in a solution, phosphate was removed from a 0.1 M NaH₂PO₄ solution (pH adjusted to 7 with NaOH). A potential of 0.6 V Ag / AgCl was applied for 5 s to remove phosphate, followed by a 25-s rest time. This rest time was to replenish phosphate at the electrode surface to ensure that more phosphate could be incorporated into the Zn electrode upon oxidation. The process of applying a 5-s oxidation voltage pulse followed by a 25-s rest time was repeated until the total cumulative oxidation time reached 2 h.

[0091] The oxidized electrode was analyzed by XRD, and the newly emerging peaks could be indexed as those of NaZnPO₄·H₂O, indicating that the Zn electrode could remove phosphate from the solution. EDS analysis showed that the Zn:P ratio was 1:~0.51. This Zn:P ratio was different from the expected 1:1 for NaZnPO₄·H₂O because not all Zn was converted to NaZnPO₄·H₂O.

[0092] For the phosphate removal step, the zinc phosphate electrode obtained from phosphate removal was immersed in a 0.01 M NaH₂PO₄ recovery solution (pH 7) and reduced back to Zn at a constant current of -16 mA / cm 2 for 1 h. The resulting electrode was analyzed by EDS, and no P was detected, indicating that all the phosphate stored in the electrode was released from the electrode and recovered into the recovery solution.

[0093] It was also found that when a ZnO flake electrode was immersed in a phosphate solution without first reducing ZnO to Zn, the same NaZnPO₄·H₂O could be spontaneously formed by non-electrochemical anion exchange (replacing oxygen ions with phosphate ions) because the conversion of ZnO to NaZnPO₄·H₂O was thermodynamically favorable. The amount of phosphate incorporated into the ZnO electrode by this non-electrochemical method was less than that achieved by applying a metal oxidation potential to a pre-reduced Zn electrode. For example, when a ZnO electrode was immersed in a 0.1 M NaH₂PO₄ solution (pH 7) for 2 h, the Zn:P ratio of the resulting electrode was 1:0.32.

[0094] However, when using a more concentrated phosphate solution (1 M NaH₂PO₄, pH 7), immersing the ZnO electrode in this solution for 10 min was sufficient to achieve a Zn:P ratio of up to 1:0.9. Through the above phosphate recovery conditions, it was confirmed that the NaZnPO₄·H₂O phase formed by the non-electrochemical method could also be reduced to Zn.

[0095] The term "exemplary" is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for purposes of this disclosure and unless otherwise specified, "a" may mean only one or may mean "one or more". Embodiments consistent with either interpretation are covered.

[0096] For purposes of illustration and description, the foregoing description of illustrative embodiments of the invention has been given. It is not exhaustive and is not intended to limit the invention to the precise forms disclosed, and modifications and variations may be made in light of the above teachings, or may be obtained from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and as practical applications of the invention to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. The scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for removing phosphorus from an aqueous solution containing phosphate ions using an electrochemical cell, the cell comprising: a first electrode in the aqueous solution, the first electrode comprising a metal, a metal oxide, or a combination thereof, wherein the metal, metal oxide, or combination thereof is selected from bismuth, bismuth oxide, zinc, zinc oxide, copper, copper oxide, iron, and iron oxide, the method comprising: converting at least a portion of the metal, metal oxide, or combination thereof in the first electrode into a metal phosphate phase in the first electrode, the metal phosphate phase comprising a metal phosphate selected from bismuth phosphate, zinc phosphate, copper phosphate, and iron phosphate; replacing the phosphate ion-containing solution with a second aqueous solution; providing an anode for phosphate recovery in the electrochemical cell; and applying a voltage across the phosphatized first electrode and the anode for phosphate recovery, wherein the voltage drives the reduction of the metal phosphate phase in the first electrode, releases phosphate ions into the second aqueous solution, and drives an oxidation reaction at the anode for phosphate recovery.

2. The method according to claim 1, wherein the electrochemical cell includes a cathode for phosphate removal, and the step of electrochemically converting at least a portion of the metal, metal oxide, or combination thereof in the first electrode into a metal phosphate phase is performed by applying a voltage across the first electrode and the cathode for phosphate removal.

3. The method according to claim 2, wherein the first electrode comprises bismuth, and the metal phosphate is bismuth phosphate.

4. The method according to claim 2, wherein the first electrode comprises copper, and the metal phosphate is copper phosphate.

5. The method according to claim 2, wherein the first electrode comprises iron, and the metal phosphate is iron phosphate.

6. The method according to claim 1, wherein the first electrode comprises zinc, zinc oxide, or a combination thereof, and the metal phosphate is zinc phosphate, and further wherein the step of converting at least a portion of the zinc, zinc oxide, or combination thereof in the first electrode into a zinc phosphate phase is performed non-electrochemically.

7. The method according to claim 2, wherein the first electrode comprises zinc, zinc oxide, or a combination thereof, and the metal phosphate is zinc phosphate.

8. The method according to claim 1, wherein the aqueous solution containing phosphate ions has an initial phosphate ion concentration of at least 1×10 -3 M.

9. The method according to claim 1, wherein the phosphate ion-containing aqueous solution has an initial phosphate ion concentration of 0.1 M to 1 M.

10. The method according to claim 3, wherein the second aqueous solution is acidic, and the phosphate ions released into the second aqueous solution form phosphoric acid.

11. The method according to claim 2, wherein a water reduction reaction is performed at the cathode for phosphate removal.

12. The method according to claim 2, wherein an oxygen reduction reaction is performed at the cathode for phosphate removal.

13. The method according to claim 2, wherein a metal ion reduction reaction is performed at the cathode for phosphate removal.

14. The method according to claim 13, wherein the metal ion is an iron ion, a zinc ion, or a copper ion.

15. The method according to claim 14, wherein the first electrode comprises bismuth, and the metal phosphate is bismuth phosphate.

16. The method according to claim 1, wherein the aqueous solution containing phosphate ions comprises metal phosphates obtained from chemical or electrochemical coagulation processes or from biological phosphate removal processes.

17. The method according to claim 13, wherein the aqueous solution containing phosphate ions comprises metal phosphates obtained from chemical or electrochemical coagulation processes.

18. The method according to claim 1, wherein water oxidation is carried out at the anode for phosphate recovery.

19. The method according to claim 2, wherein the cathode for phosphate removal and the anode for phosphate recovery are different electrodes.

20. The method according to claim 2, wherein the cathode for phosphate removal and the anode for phosphate recovery are the same electrode.

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