Spontaneous Generation of H2O2 and Hydroxyl Radical through O2

Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Spontaneous Generation of H2O2 and Hydroxyl Radical through O2 Reduction on Copper Phosphide under Ambient Aqueous Condition Hyejin Kim,† Jonghun Lim,† Seonggyu Lee,† Hak-Hyeon Kim,‡ Changha Lee,§ Jinwoo Lee,∥ and Wonyong Choi*,†

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†

Division of Environmental Science and Engineering & Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea ‡ School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea § School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea ∥ Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea S Supporting Information *

ABSTRACT: Copper phosphide (CuxP) was synthesized and tested for its reactivity for generating H2O2 through spontaneous reduction of dioxygen under ambient aqueous condition. The in situ generated H2O2 was subsequently decomposed to generate OH radicals, which enabled the degradation of organic compounds in water. The oxygen reduction reaction proceeded along with the concurrent oxidation of phosphide to phosphate, then copper ions and phosphate ions were dissolved out during the reaction. The reactivity of CuxP was gradually reduced during 10 cycles with consuming 8.7 mg of CuxP for the successive removal of 17 μmol 4-chlorophenol. CoP which was compared as a control sample under the same experimental condition also produced H2O2 through activating dioxygen but did not degrade organic compounds at all. The electrochemical analysis for the electron transfers on CuxP and CoP showed that the number of electrons transferred to O2 is 3 and 2, respectively, which explains why OH radical is generated on CuxP, not on CoP. The Cu+ species generated on the CuxP surface can participate in Fenton-like reaction with in situ generated H2O2. CuxP is proposed as a solid reagent that can activate dioxygen to generate reactive oxygen species in ambient aqueous condition, which is more facile to handle and store than liquid/gas reagents (e.g., H2O2, Cl2, O3).

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INTRODUCTION Advanced oxidation processes (AOPs) have been intensively studied for efficient removal of recalcitrant organic contaminants with high chemical stability or low biodegradability in wastewater.1−4 For energy-sustainable applications of AOPs, it is essential to develop a system that efficiently produces reactive oxygen species (ROS) such as hydroxyl radical and superoxide anion. Various methods such as photolysis, photocatalysis, electrolysis, and sonolysis have been developed to efficiently produce ·OH from oxygen or water. However, these systems are often costly because external energy input (e.g., UV light, electricity, and ultrasound) is required. From the environmental point of view, the spontaneous formation of ROS through ambient activation of dioxygen without the need of external energy is highly desirable. Some examples of O2 activation via H2O2 formation have been reported.5−8 An example is to activate dioxygen by utilizing the redox chemistry of zerovalent-iron (ZVI) which is earth-abundant, nontoxic, and inexpensive. The corrosive oxidation of Fe0 to Fe2+ (E0(Fe2+/Fe0) = −0.447 VNHE) can © XXXX American Chemical Society

be coupled with two-electron reduction of dioxygen to generate H2O2 (E0(O2/H2O2) = +0.695 VNHE).5,6 The in situ generated H2O2 is then activated by Fenton reaction producing hydroxyl radical. However, ZVI has a limited reactivity because of the presence of native oxide layer on its surface. A similar process can be achieved in a homogeneous solution which utilizes Cu(I) for in situ H2O2 generation by reducing O2 under neutral pH condition. Then, H2O2 can be decomposed to hydroxyl radical by Cu(I) via Fenton-like reaction or can oxidize Cu(I) into Cu(III) as an active oxidant.7,8 To facilitate the regeneration of Cu(I), hydroxylamine could be used as a reducing agent of Cu(II). However, hydroxylamine is highly toxic and not suitable to be used as a reagent for water treatment. Received: November 10, 2018 Revised: January 12, 2019 Accepted: February 12, 2019

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DOI: 10.1021/acs.est.8b06353 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Commercial ZVI. Iron powder (100 mesh) as another control sample was purchased from Fischer Scientific for the comparison with CuxP. Characterization of Materials. The structure of materials was analyzed by powder X-ray diffraction (XRD) using an Xray diffractometer (RIGAKU D MAX 2500) with Cu Kα radiation. The surface chemical compositions were examined by X-ray photoelectron spectroscopy (XPS, VG Escalab 250) with a monochromatic Al Kα line (1486.6 eV) as an excitation source. The high-resolution transmission electron microscopy (HR-TEM) and electron energy loss spectra (EELS) mapping were performed on a JEM-2200F microscope (JEOL) with the Cs-corrected line. Batch Reaction Tests. The prepared sample was dispersed in distilled water or wastewater at a concentration of 0.5 g/L under ultrasonication. An aliquot of substrate stock solution (4-chlorophenol (4-CP), 1,4-dioxane, bisphenol A, trimethoprim) was added to make a desired concentration of each target pollutant. A wastewater (WW) sample was obtained from an industrial wastewater treatment plant (Pohang, Republic of Korea) and the composition was analyzed (see Supporting Information (SI) Table S1). The suspension pH was controlled by HClO4 or NaOH solution. The reaction was carried out in air-equilibrated suspension (30 mL) in a beaker with continuous stirring. For the recycling test, 15 mg of copper phosphide was dispersed in 30 mL of DI water containing 4-CP (0.1 mM) at pHi 3. After reaction, the suspension was filtered to recover the copper phosphide sample which was subsequently washed and then redispersed in a fresh substrate solution. This procedure was repeated for 10 cycles. Analyses. The concentrations of 4-CP, bisphenol A, and trimethoprim were analyzed by a high performance liquid chromatograph (HPLC, Agilent 1260 Infinity) equipped with a diode array detector and a ZORBAX 300SB-C18 column (4.6 mm × 150 mm). For the analysis of 1,4-dioxane, Bondapak C18 column (Waters, 10 μm, 3.9 × 300 mm) was used. The HPLC measurement of 4-CP, bisphenol A was carried out using a binary mobile phase of 85% (v/v) aqueous phosphoric acid solution and acetonitrile (70:30 by volume). The concentration of trimethoprim was analyzed using a mobile phase of 25 mM ammonium acetate and acetonitrile (80:20 by volume) with the detection wavelength at 230 nm and 1,4dioxane was analyzed using water eluent containing 5% acetonitrile with the detection wavelength at 190 nm. The total organic carbon (TOC) was monitored using a TOC analyzer (TOC-LCPH, Shimadzu). The concentration of anions was analyzed using an ion chromatograph (IC, Dionex DX-120) equipped with a conductivity detector and an AS-14 (4 mm × 250 mm) column using the eluent composition of 3.5 mM Na2CO3/1 mM NaHCO3. The concentration of Cu(I) was determined by the neocuproine method. The Cu(II) ion was reduced to Cu(I) by 10 wt % of hydroxylamine (50 wt % in H2O, Aldrich) and then analyzed by the neocuproine method.8 The leached amounts of Cu, Co, and P from CuxP and CoP were measured by inductively coupled plasma optical emission spectrometry (ICP-OFS-6300-Thermo scientific). The in situ generated H2O2 was measured by the colorimetric method using N,N-diethyl-1,4-phenylene-diamine sulfate.22 The production of hydroxyl radical was indirectly monitored by the fluorescence method using a spectrofluorometer (HORIBA, Fluoromax 4C-TCSPC). Coumarin was

Similar to ZVI, zerovalent copper (ZVC) has been also used for AOP as an activator of H2O2 and persulfate (PS)/ peroxymonosulfate (PMS). The Cu(I) released from ZVC dissolution can activate H2O2 (with the assistance of ultrasound) or PS/PMS to generate hydroxyl radical and sulfate radical.9,10 ZVI with copper loading as bimetallic nanoparticles can activate periodate to generate iodyl radical as a predominant oxidant.11 However, these systems need external energy and/or chemical oxidants as a precursor of oxidant radicals. The heterogeneous Fenton-like reaction using Cu mostly utilized the mixed oxides of Fe and Cu (CuFe2O4 and CuFeO2):12,13 Fe3+ and Cu2+ on the surface of catalyst react with H2O2 and the resulting Cu+ and Fe2+ induce Fenton-like reaction generating hydroxyl radical and Cu+ reduces Fe3+ back to Fe2+. However, this system also needs H2O2 as a reagent. The heterogeneous Fenton-like reaction with in situ production of H2O2 might be enabled by employing suitable reactive minerals containing Fe and/or Cu. We note that the corrosion of mineral schreibersite ((Fe,Ni)3P) was studied to determine the origin of phosphorylated biomolecules (e.g., DNA and phospholipids):14−16 this mineral is decomposed with oxidizing phosphorus species and reducing water concurrently. In this study, we employed copper phosphide to induce dioxygen reduction instead of water reduction, which may enable Fenton-like reaction in the absence of externally added H2O2. While metal phosphides have been often used as a catalyst or cocatalyst for H2 evolution,17,18 their application to water treatment has not been investigated. This study found that copper phosphide generates in situ H2O2 via the reduction of O2 and subsequently produces OH radicals in ambient condition to degrade organic pollutants in water. The characterization of copper phosphide and the reactions and mechanisms of hydroxyl radical generation are described and discussed in detail.

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EXPERIMENTAL SECTION Material Synthesis. Preparation of CuxP (x = 3, 1/2). Copper Phosphide that is composed of Cu3P and CuP2 was prepared as follows. A mixture of 0.1 g of Cu(OH)2 (Aldrich) and 0.6 g of NaH 2 PO 2 ·H 2 O (Aldrich) were ground homogeneously. The mixture was placed in a crucible boat with a cover and then calcined at 300°C for 2 h with a heating rate of 2 °C min−1 under Ar atmosphere in a tube furnace. The phosphine (PH3) generated in situ from the thermal decomposition of sodium hypophosphite react with copper hydroxide to make copper phosphide.19,20 After the resulting copper phosphide sample was ground, CuxP was dispersed in 200 mL of deionized (DI) water and then sonicated in an ultrasonic bath (JAC 4020, 400 W, Sonic) for 1.5 h, filtered, washed with DI water and dried at 80 °C to obtain purified samples. Preparation of CoP. CoP (as a control sample to be compared with CuxP) was synthesized in the same way of CuxP synthesis. One difference is that Co(OH)2 precursor was synthesized by a precipitation method.21 A solution of NaOH (20 mL, 0.25 M, Aldrich) was added to the solution of Co(NO3)2·6H2O (50 mL, 0.05 M, Aldrich) dropwise under magnetic stirring. After 2 h, the precipitates (Co(OH)2 (s)) were filtered, washed with DI water and ethanol, and then dried at room temperature. The remaining synthesis procedure was the same to that of CuxP. B


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Environmental Science & Technology used as a chemical trap of ·OH, which is oxidatively converted into 7-hydroxycoumarin through its reaction with ·OH.23 The fluorescence emission intensity of 7-hydroxycoumarin was measured at 460 nm under the excitation at 332 nm. Terephthalic acid (TA) was also used as a trap of ·OH which generates a hydroxylated product.23 Fluorescence emission of 2-hydroxyterephthalic acid (2-HTA) was monitored at 425 nm under the excitation at 315 nm. For electron paramagnetic resonance (EPR) analysis for ·OH detection, 5,5dimethyl-1-pyrroline N-oxide (DMPO) was used as a spintrapping agent.24 The EPR signals of the DMPO−OH spin adduct were monitored using a EPR spectroscopy (JES-X310, Jeol Co.) under the following conditions: microwave power, 1.00 mW; microwave frequency, 9.42 GHz; modulation frequency, 100 kHz; modulation amplitude, 2.0 G. Electrochemical Analysis. The oxygen reduction reaction (ORR) occurring on CuxP and CoP was investigated by linear sweep voltammetry (LSV) method (LSV; CHI 660E potentiostat) in O2-purged 0.1 M KOH solution. The working electrode was prepared as follows. The solvent of water and 2propanol (volume ratio = 1:1) which contained Nafion ionomer was prepared for dispersing the catalyst. Nafion ionomer was used as a binder for catalyst with the mass ratio of 1:0.5 (Nafion ionomer to catalyst). The prepared catalyst slurry was loaded on a glassy carbon disk electrode with the catalyst loading of 255 μg/cm2. An Hg/HgO (in 1 M NaOH) electrode and a platinum wire were used as a reference electrode and a counter electrode, respectively. LSV was obtained with a scan rate of 10 mV/s from 0.0 to 0.7 V at different rotating speeds. The electron transfer number was determined from the slope of Koutecky−Levich (KL) plot that exhibits a linear correlation between j−1 and ω−1/2 (j: current density, ω: angular velocity of the disk electrode).25

Figure 1. (a) XRD spectra of CuxP. (b) XPS spectra of CuxP in Cu 2p and (c) P 2p binding energy region. (d) TEM images of CuxP with EELS mapping corresponding to copper (yellow) and phosphorus (blue) elements.

bonded to Cu) and phosphate species (P2O5 or PO43−) in the oxidized surface region of copper phosphide, repectively.31,32 The P content of the phosphate species was estimated to be 59% of the total P species in CuxP from the XPS analysis, which indicates that the surface of CuxP is covered with an oxidized layer. The elemental distributions in the copper phosphide was confirmed by HR-TEM and EELS analysis (Figure 1d), which shows aggregated nanoparticles of copper phosphide with well distributed elements of Cu and P. Cobalt phosphide was chosen for comparison with copper phosphide because of its similar property of oxygen reduction reaction and was synthesized as a control material.21 SI Figure S1a shows the XRD patterns of CoP. All XRD peaks match the simulated XRD peaks of CoP. The Co 2p spectra for the cobalt phosphide in SI Figure S1b exhibits Co 2p3/2 and 2p1/2 with satellite peaks. The peaks at 782.1 and 798.1 eV correspond to the oxidized Co species (Co2+). The peak at 778.5 and 793.9 eV represent partially oxidized Co species (cf. metallic Co binding energy reference: 777.9 eV) bound with P species, which indicates that Co in CoP has the oxidation state in the range of 0 < x < + 2.33,34 SI Figure S1c shows the P 2p peaks at 129.6 eV (P bonded to Co) and 133.6 eV in phosphate species.35 The TEM and EELS images of SI Figure S1d show aggregated cobalt phosphide composed of uniformly distributed cobalt and phosphorus elements. Spontaneous Formation of Reactive Oxygen Species on Copper Phosphide. When CuxP was dispersed in acidic water, H2O2 was spontaneously generated within a minute as shown in Figure 2a. The dissolved oxygen should be the precursor of H2O2 production (Reaction 1) since H2O2 was not generated at all under Ar-saturated condition.

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RESULTS AND DISCUSSION Characterization of Materials. The XRD was measured to investigate the crystal structures of synthesized copper phosphide, which exhibited the diffraction peaks for Cu3P and CuP2 (Figure 1 a). The red circles represent the reflections of (112), (202), (211), (300), (113), (212), (104), (222), and (214) planes, which can be assigned to hexagonal phase of Cu3P.26 The blue triangles indicate the diffraction peaks of crystalline monoclinic of CuP2, which consists of (011), (−112), (200), (−113), (−204), (−222), (300), (310), (−131), and (−115) planes.27 The two peaks corresponding to (−211) and (022) planes of CuP2 overlapped with (112) and (113) planes of Cu3P. All the XRD peaks of the synthesized copper phosphide coincided with those of simulated XRD spectra of monoclinic CuP2 and hexagonal Cu3P. The oxidation state of Cu was analyzed by X-ray photoelectron spectroscopy (XPS) analysis. Figure 1b shows the deconvoluted three components of Cu 2p peak in copper phosphide. The binding energy bands around 934 and 954 eV region are assigned to the characteristic peaks of Cu 2p3/2 and 2p1/2 respectively. The peaks at 932.5 and 952.4 eV are assigned to Cu+ in CuxP and those at 934.9 and 954.8 eV to Cu2+ states.28 The two satellite peaks located at 942.8 and 962.6 eV are attributed to the +2 oxidation state.29,30 The Cu2+ state shown in XPS might be ascribed to the oxidized copper species on the copper phosphide surface region which is in contact with air. The Figure 1c exhibits the P 2p peaks at 129.6 and 133.6 eV, which are assigned to the reduced P species (P

O2 + 2e− + 2H+ → H 2O2

(1)

However, the concentration of in situ generated H2O2 gradually decreased with reaching a plateau. This implies that H2O2 further reacted on CuxP and approached a steadystate where the in situ production and decomposition of H2O2 is balanced. The production of H2O2 with CoP was also tested as a control under the same condition. Figure 2b C


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This further supports that H2O2 is a precursor of ·OH in CuxP system. H 2O2 + e− + H+ → ·OH + H 2O

(4)

On the other hand, 7-HC and 2-HTA were not generated at all with CoP although CoP produced much higher concentration of H2O2 than CuxP. This indicates that both CuxP and CoP reductively transform O2 to H2O2 but they differ in further reactions with H2O2. To test the reactivity of CuxP and CoP with H2O2, the degradation of H2O2 was compared in the Arpurged suspension of CuxP and CoP (Figure 2e). H2O2 was rapidly decomposed in the presence of CuxP, but not degraded at all in the presence of CoP. This is fully consistent with the observation that CuxP decomposed in situ generated H2O2 to produce ·OH but CoP did not induce the production of ·OH (Reaction 5) whereas O2 reduction on CoP should stop after two electron transfer (Reaction 1). O2 + 3e− + 3H+ → ·OH + H 2O

(5)

The in situ generated ·OH on CuxP can be used for degrading organic pollutants. 4-CP decomposition on CuxP along with the stoichiometric production of chloride ions was successfully demonstrated (Figure 3a). The degradation

Figure 2. (a) H2O2 generation on CuxP and (b) CoP under different dissolved gas and pH conditions. ([CuxP or CoP] = 0.5 g/L) (c) Production of 7-hydroxycoumarin (coumarin−OH adduct) and (d) 2-hydroxyterephthalic acid (terephthalic acid−OH adduct) as a probe of OH radicals. ([CuxP or other reagent] = 0.5 g/L; [Coumarin]0 = 1 mM; [TA]0 = 0.5 mM; pH 3.0; air-equilibrated (unless indicated)) (e) H2O2 decomposition in Ar-saturated condition. ([CuxP] = 0.5 g/ L; [H2O2]0 = 1 mM; pH 3.0).

shows that more H2O2 was produced on CoP and it continued to increase without showing any sign of decrease unlike CuxP. Note that both CuxP and CoP exhibited highly enhanced production of H2O2 (@ pH 3) in the O2-saturated condition. The production rate of H2O2 was higher at lower pH for both samples. The fact that H2O2 production is favored with higher concentrations of dissolved O2 and protons reassures that H2O2 is produced via Reaction 1 in the suspension of both CuxP and CoP. One difference between CuxP and CoP is that CuxP can induce further reaction of H2O2. To confirm whether the in situ production of H2O2 induces the generation of hydroxyl radical (·OH), coumarin and terephthalic acid (TA) was employed as a probe of ·OH to monitor the production of 7-hydroxycoumarin (7-HC) and 2hydroxyterephthalic acid (2-HTA) through Reaction 2 and 3, respectively. ·

OH + Coumarin → → 7‐hydroxycoumarin

(2)

·

OH + terephthalic acid → → 2‐hydroxyterephthalic acid

(3)

Figure 3. (a) Time profiles of 4-CP degradation with the concurrent generation of chloride in CuxP suspension. (b) Effect of CuxP loading on the degradation of 4-CP. (c) Comparison of 4-CP degradation efficiencies among CuxP, CoP, and ZVI systems. ([CuxP or other reagent] = 0.5 g/L; [4-CP]0 = 100 μM; pH 3.0; air-equilibrated) (d) Various organic pollutants (C0 = 300 μM) degradation on CuxP.

efficiency of 4-CP increased with raising the loading of CuxP up to 0.5 g/L beyond which the reactivity was saturated as shown in Figure 3b. It seems that the overall reactivity of CuxP is limited by the mass transfer of O2 molecules onto the surface of CuxP in the presence of enough solid reagent. CoP that did not produce OH radicals could not degrade 4-CP at all under the same experimental condition (Figure 3c). ZVI exhibited a lower removal efficiency than CuxP probably because the presence of native oxide layer on ZVI surface should retard the interfacial redox reaction. CuxP can remove 32% of TOC after 2 h reaction in DI solution. Diluted wastewater was also treated with CuxP, which achieved 73% TOC removal efficiency in 6 h reaction ([CuxP] = 0.5 g/L, pH 3, TOC0 = 7.1 ppm), which implies that various organic substances in

Figure 2c and d demonstrate that the ·OH production in CuxP solution is apparent judging from the immediate production of 7-HC and 2-HTA within a few minutes. The generation of 7HC and 2-HTA was completely inhibited under the O2depleted condition under Ar-purging. This indicates that the in situ generated H2O2 is decomposed to generate ·OH through Fenton-like electron transfer on CuxP (Reaction 4). When H2O2 was externally added to the suspension of CuxP, the production of 2-HTA was markedly enhanced (Figure 2d). D


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reaction. Since CuxP is a solid reagent that is consumed during reaction, the activity of CuxP was gradually reduced as the reaction cycle was repeated up to 10 times (Figure 4c). After 10 cycles, 58% of total CuxP was consumed for the removal of 17 μmol 4-CP, which indicates that 1.95 mmol (0.25 g) of 4CP can be degraded by consuming 1 g CuxP. In addition, the in situ deposition of copper phosphate on CuxP surface which might occur during the repeated reaction cycles can be also responsible for the gradual deactivation of CuxP, although the CuxP remaining after the initial 2 h reaction retained its crystallinity and the oxidation states of Cu and P (see SI Figure S3). Reaction Mechanisms of Copper Phosphide. The spontaneous production of H2O2 and ·OH in CuxP suspension is a result of the oxidative dissolution of CuxP with accompanying the generation of cuprous, cupric and orthophosphate ions as Figure 5a and b show. The oxidative

wastewater can be mineralized by CuxP-induced oxidation. Other organic contaminants that contain different chemical functional groups such as ether, phenol or pyrimidine were also chosen and tested for their degradation in the presence of Cux P. These contaminants could be also successfully decomposed by CuxP (Figure 3d). The oxidation efficiency using CuxP (for the removal of 4CP) was highly pH-dependent and markedly decreased in near neutral and basic conditions as shown in Figure 4a. This is

Figure 5. Time−concentration profiles of (a) cuprous, cupric and (b) orthophosphate ions dissolved out from CuxP. (c) Effects of scavengers (TBA and POD) and Ar-purging on 4-CP degradation. ([CuxP] = 0.5 g/L; [4-CP]0 = 100 μM; [TBA]0 = 0.1 M; [POD] = 100 mg/L; pH 3.0; air-equilibrated) (d) Removal of Cr(VI) with the concurrent gerneration of orthophosphate in Ar-purged suspension of CuxP. ([CuxP] = 0.5 g/L; [Cr(VI)]0 = 200 μM; pH 3.0).

Figure 4. (a) pH-dependent degradation of 4-CP on CuxP and (b) The concentration of Cu and P species leached out from CuxP after 2 h reaction. (c) Ten repeated cycles of 4-CP degradation at pH 3 in CuxP suspension. ([CuxP] = 0.5 g/L; [4-CP]0 = 100 μM; airequilibrated).

because the high pH condition does not favor the production of H2O2 which is the precursor of ·OH (see Figure 2a). From the ICP analysis, it was confirmed that the copper and phosphorus species were leached out during the reaction of CuxP (Figure 4b). About 40% of phosphorus detected by ICP analysis corresponded to orthophosphate ions which were identified by IC analysis. The remaining phosphorus species might exist as polyphosphates that are generated through further reactions of phosphate.36 Based on the measured concentration of copper and phosphorus leached out, it was estimated that the 17.6% of CuxP was dissolute (or consumed) in 2 h reaction. The fact that CuxP is more reactive for the substrate degradation at acidic pH condition indicates that the oxidative reaction induced by CuxP proceeds through its oxidative dissolution coupled with the production of ·OH which requires the proton supply (Reaction 5). CuxP should be an oxidative reagent (not catalyst) that generates OH radical with slowly dissolving itself in ambient aqueous solution. Likewise, CoP leached cobalt and phosphorus species in acid condition (see SI Figure S2) along with the production of H2O2. About 22% of CoP was consumed at pH 3 in 5 h

dissolution of metal phosphide was also reported for Fe3P, which generated phosphate and phosphite ions in acidic solution (Reaction 6).16 10H 2O + 3Fe3P + 15H+ → 9Fe2 + + H 2PO4− + 2H 2PO3− + 14.5H 2

(6)

Likewise, CuxP can be oxidatively dissoluted to phosphate and phosphite as dissolved O2 is reduced to H2O2. In this case, phosphite was not detected by IC analysis. It seems that phosphite, if any, is rapidly oxidized to phosphate by ·OH.15 This implies that phosphite ions generated as an intermediate of oxidative dissolution of CuxP might serve as an in situ scavenger of hydroxyl radicals to limit the action of ·OH. The dissolved copper and phosphate ions could be readily removed from the solution by precipitation with adjusting the pH.8,37 The formation of tenorite (CuO) from the precipitation of Cu(II) is favored at pH > 6 and Cu3(PO4)2 is easily precipitated in the presence of phosphate around neutral pH (see SI Figure S4). Polyphosphate could be also precipitated by binding copper ions.38 The concentration of dissolved E


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The overall mechanism of CuxP seems to proceed through two sequential steps: (1) the oxidative dissolution of CuxP accompanying the O2 reduction to H2O2 and (2) Fenton-like reaction of in situ generated H2O2 to generate ·OH. Therefore, the degradation of 4-CP was completely inhibited in the absence of O2 (Ar-purged) and in the presence of scavengers of H2O2 (POD, horseradish peroxidase) or ·OH (TBA) (see Figure 5c). This supports that ·OH generated from O2 reduction via H2O2 generation on CuxP is responsible for the degradation of organic pollutants. The production of ·OH was also confirmed by its detection using EPR spin-trapping technique, which is shown in Figure 6b. The EPR peaks of DMPO−OH were detected only in the presence of both O2 and CuxP, which reconfirms that ·OH comes from the reductive conversion of O2 on CuxP. On the other hand, CoP that efficiently generated H2O2 in the presence of O2 generated no DMPO−OH signal, which is consistent with its inability to degrade organic compound (see Figure 3c). Although CuxP is inactive for oxidation in the absence of O2, it can be active in the presence of alternative electron acceptor such as hexavalent chromium (Cr(VI)). Figure 5d shows that CuxP in Ar-purged suspension could quickly reduce Cr(VI) to Cr(III) with the concurrent generation of phosphate. This confirms that CuxP can be oxidatively dissolved to produce phosphate ions even in the absence of O2 when there are alternative electron acceptors. This implies that the oxygen elements in phosphate come from water, not dioxygen. Incidentally, it is interesting to note that the phosphate ions generated in the presence of Cr(VI) decreased gradually after the rapid production in the initial period unlike the case of phosphate generation in the absence of Cr(VI) (compare Figure 5b and d). This can be ascribed to that phosphate and Cr(III) form insoluble complexes.41,42 Environmental Applications. This study demonstrated the production of ROS from the oxygen reduction reaction coupled with copper phosphide oxidation in ambient aqueous condition without using external energy and additional oxidants. The production of ·OH from the in situ formed H2O2 enabled the degradation of recalcitrant organic pollutants. As a result of the redox reaction of CuxP, copper and phosphate ions were dissolute into the solution, which may cause an additional pollution in the treated water. However, a pH adjustment to pH 7 rapidly removed copper and phosphate ions via precipitation. CuxP as a stable solid reagent can be utilized as an efficient and easily manageable chemical for water treatment, which can produce in situ ·OH in ambient conditions. The proposed CuxP process can be similarly compared with conventional Fenton process in some aspects. First, the CuxP reaction is largely limited at acidic condition and needs neutralization after treatment like Fenton process. Second, both systems leave solid wastes such as copper phosphate and iron hydroxide. Copper phosphate is more hazardous than iron-containing sludge, which may limit its widespread applications. However, as long as copper phosphate can be recovered and reprocessed to regenerate copper phosphide reagent, the hazard of the Cu-containing sludge can be controlled. On the other hand, the biggest advantage of CuxP process is no need of H2O2 reagent which accounts for the largest portion of the chemical cost of Fenton process. In addition, CuxP is a stable solid reagent that can be easily transported and stored before use whereas liquid H2O2 that is unstable and highly caustic is not suitable for easy handling and long-term storage. Such stable solid reagents for

copper and phosphorus ions was 1.21 mM and 0.56 mM after 2 h reaction of CuxP (with [4-CP]0 = 100 μM) at pH 3. After the reaction, the suspension containing the remaining CuxP was neutralized to pH 7 and the concentration of dissolved copper and phosphate ions was immediately reduced to 45 μM and 38 μM, respectively. Cu and P ions that were generated as byproducts of CuxP reaction could be readily removed (>94% removal efficiency) by the pH-controlled precipitation. To investigate the activation of O2 via the electron transfer on CuxP, the number (n) of electrons transferred in the oxygen reduction reaction (ORR) was estimated by the rotating disk electrode (RDE) analysis. The KL plots in Figure 6a show that

Figure 6. (a) Koutecky−Levich plots of CoP and CuxP electrode obtained from RDE measurment with continuous O2 purging at 0.2 V (vs RHE). (b) EPR spectra of OH-DMPO adducts formed in CuxP or CoP suspension ([CuxP or CoP] = 1 g/L; [DMPO]0 = 10 mM; pH 3.0; air-equilibrated).

the n value of CuxP/O2 and CoP/O2 systems was estimated to be about 3 and 2, respectively. This means that CoP preferentially transfers two electrons to dioxygen to generate H2O2 (Reaction 1) whereas CuxP transfers 3 electrons to O2 to produce ·OH via H2O2 (Reaction 5). What makes the behaviors of CuxP and CoP different should be related with the reaction of in situ generated H2O2 with the metal ions. Since CuxP contains Cu+ species on the surface, Cu+ induces Fenton-like reaction with in situ generated H2O2 to generate OH radical (Reaction 7).39,40 As a result of this reaction, Cu+ in CuxP is oxidized to Cu2+, which was leached out from the CuxP lattice (Figure 5a). Cu+ + H 2O2 → Cu 2 + + ·OH + OH− [E 0(H 2O2 / ·OH) = 1.14 VNHE vs E 0(Cu 2 +/Cu+) = 0.153 VNHE]

(7)

However, such Fenton-like reaction cannot be induced on CoP since it is not thermochemically allowed based on the standard reduction potential of E0(Co3+/Co2+) = 1.82 VNHE. F


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water treatment are few but very convenient for practical applications. Although the use of Cu-containing compound as a water treatment reagent has an intrinsic limitation, CuxP that consists of cheap and abundant elements can be potentially useful for wastewater treatment without the need of external energy and hazardous liquid/gas reagents (e.g., H2O2, Cl2, O3). In particular, acidic and heavy metal polluted wastewaters from steel industry, mining, and electroplating industry can be effectively treated by the CuxP method since not only organic pollutants but also heavy metal ions can be removed by adsorption or coprecipitation when heavy metal ions and in situ generated phosphates form insoluble precipitates. In addition, adding H2O2 can further promote the oxidative reactivity of CuxP as in conventional Fenton treatment systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b06353. XRD, XPS, and TEM analysis of CoP (Figure S1), pHdependent dissolution of CoP (Figure S2), CuxP samples analyzed before and after the reaction (Figure S3), the pH-dependent speciation of Cu(II) (Figure S4) and the tested wastewater composition (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-54-279-2283; e-mail: [email protected]. ORCID

Jonghun Lim: 0000-0002-2943-1846 Changha Lee: 0000-0002-0404-9405 Jinwoo Lee: 0000-0001-6347-0446 Wonyong Choi: 0000-0003-1801-9386 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the Global Research Laboratory (GRL) Program (No. NRF-2014K1A1A2041044) and Basic Science Research Program (NRF2017R1A2B2008952), which were funded by the Korea government (MSIP) through the National Research Foundation (NRF).

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REFERENCES

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Spontaneous Generation of H2O2 and Hydroxyl Radical through O2

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