Activation of Peroxymonosulfate by Oxygen Vacancies-Enriched Cobalt-Doped Black TiO2 Nanotubes for the Removal of Organic Pollutants

Cite this: Environ. Sci. Technol. 2019, 53, 12, 6972–6980
Publication Date (Web):May 15, 2019
https://doi.org/10.1021/acs.est.9b01449
Copyright © 2019 American Chemical Society
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Abstract

Cobalt-mediated activation of peroxymonosulfate (PMS) has been widely investigated for the oxidation of organic pollutants. Herein, we employ cobalt-doped Black TiO2 nanotubes (Co-Black TNT) for the efficient, stable, and reusable activator of PMS for the degradation of organic pollutants. Co-Black TNTs induce the activation of PMS by itself and stabilized oxygen vacancies that enhance the bonding with PMS and provide catalytic active sites for PMS activation. A relatively high electronic conductivity associated with the coexistence of Ti4+ and Ti3+ in Co-Black TNT enables an efficient electron transfer between PMS and the catalyst. As a result, Co-Black TNT is an effective catalyst for PMS activation, leading to the degradation of selected organic pollutants when compared to other TNTs (TNT, Co-TNT, and Black TNT) and other Co-based materials (Co3O4, Co-TiO2, CoFe2O4, and Co3O4/rGO). The observed organic compound degradation kinetics are retarded in the presence of methanol and natural organic matter as sulfate radical scavengers. These results demonstrate that sulfate radical is the primary oxidant generated via PMS activation on Co-Black TNT. The strong interaction between Co and TiO2 through Co–O–Ti bonds and rapid redox cycle of Co2+/Co3+ in Co-Black TNT prevents cobalt leaching and enhances catalyst stability over a wide pH range and repetitive uses of the catalyst. Electrode-supported Co-Black TNT facilitates the recovery of the catalyst from the treated water.

Introduction

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Peroxymonosulfate (PMS)-activated oxidation has been investigated for water treatment and soil remediation through nonradical and radical pathways. (1−5) A nonradical mechanism involves the direct electron transfer from organic electron donor to PMS on catalyst surfaces (e.g., carbon nanotubes (CNT) and reduced graphene oxide, rGO) leading to the oxidation of organic compounds with the formation of sulfate (E0(HSO5/SO42–) = 1.75 VNHE). (6−8) On the other hand, free radical pathways involve the formation of sulfate radical anions (E0(HSO5/SO4 –) = 2.43 VNHE) generated from PMS by catalysts coupled with an external energy input to cleave the peroxide bond. (9) The free-radical mechanism has attracted more attention, compared to the nonradical mechanism, because it can treat a broad spectrum of recalcitrant organic pollutants, given the high oxidizing potential of SO4 – over a broad range of pH. (10) A variety of methods have been employed to generate SO4 – via PMS activation. (11−19) Transition-metal ions with multiple valence state are most frequently used for activation of PMS without the need for external energy inputs (e.g., UV light and electricity). (20)
Cobalt ion (Co2+) is often used as a PMS activator over a wide range of pH. (1) However, concerns have been raised about the use of Co2+ in water remediation, because of various human health concerns (e.g., asthma, pneumonia, and other lung problems). (21,22) As a result, heterogeneous cobalt-based catalysts have been developed to avoid the potential problems of using homogeneous solutions of soluble cobalt. For example, Co3O4 (20 nm)/PMS has been shown to be better than homogeneous Co2+/PMS activation for organic compound degradation, in part, because there is minimal leaching of Co2+ from Co3O4 at circum-neutral pH. (23,24) Other heterogeneous Co-containing materials have been proposed for PMS activation such as CoxFe3–xO4 and CoxMn3–xO4 without Co2+ leaching, because of the strong interactions between Co and the doped transition-metal ions. (25,26) When Co-based catalysts have been embedded into the surface of supports such as activated carbon, rGO, SBA-15, and metal organic frameworks (MOFs) PMS activation efficiencies and stabilities were improved. (27−29) Even though many Co-based catalysts have been developed to activate PMS, the practical applications of heterogeneous slurry systems are often limited by the need for catalysts recovery step. Catalysts deposited on suitable substrates (e.g., glass, silicon, and metal foams) can be easily recovered and reused without the need for physical separation and recovery step such as filtration, centrifugation, or magnetic recovery. (30,31) However, these methods are often costly and limited for long-term usage, because of the use of adhesive materials or organic binders that limit the contact between catalysts and target substrates.
TiO2 nanotube arrays (TNTs) have been used for photocatalytic and electrochemical applications, because of their fundamental properties, such as high surface areas and open-channel structures that facilitate mass transfer of the target substrates. (32,33) In addition, TNTs are synthesized on the surface of Ti-metal plates by anodization without the need for adhesives or organic binders. (34) Despite these advantages, the applications of TNTs are often restricted by their low electrical conductivity. (35) The electrical conductivity of TNTs can be enhanced by self-doping that induces a partial reduction of Ti4+ to Ti3+, together with the formation of oxygen vacancies. (36) Therefore, the resultant Black TNTs have been studied for various applications. (37) Black TNTs have some desirable properties that include high surface areas, open-channel structures, high electrical conductivity, chemical stability, easy to synthesize, and readily immobilization on support matrices.
In this study, we employed cobalt-doped Black TiO2 nanotubes (Co-Black TNT) as an efficient, stable, and reusable PMS activator for the degradation of organic pollutants. Co-Black TNT is shown to outperform other TNTs composites (bare TNT, Co-TNT, and Black TNT) and other Co-based activators (e.g., Co3O4, Co-TiO2, CoFe2O4, and Co3O4/rGO) for PMS activation. The mechanism of PMS activation is investigated using probe reagents and electron paramagnetic resonance (EPR). We also discuss the practical merits of directly growing Co-Black TNT on Ti plates without the need of additional adhesive materials or organic binders. The attached growth feature allows for the reuse of the catalysts without separation and recovery steps.

Materials and Methods

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Chemicals and Materials

Chemical reagents used in this study were as follows: potassium peroxymonosulfate (2KHSO5·KHSO4·K2SO4, available as OXONE, PMS; Aldrich), 4-chlorophenol (4-CP, Sigma–Aldrich), bisphenol A (BPA, Aldrich), phenol (J.T. Baker), benzoic acid (BA, EMD Millipore), methanol (J.T. Baker), tert-butanol (t-BuOH, Aldrich), phosphate buffer solution (pH 7.2, Sigma–Aldrich), 5-tert-butoxycarbonyl 5-methyl-1-pyrroline-N-oxide (BMPO, ENZO Life Sciences, Inc.). All chemical reagents were used as received without any purification. Deionized water (DW) was used as a solution and prepared using a Millipore system (≥18 MΩ, Milli-Q). Self-organized amorphous TNT electrodes were fabricated and simultaneously immobilized on a Ti plate by an electrochemical anodization method. The growth TNTs on a Ti-metal plate proceeds as follows: (i) formation of nonconductive thin Ti oxide layer on a Ti plate, (ii) an anodic oxidation of Ti (Ti → Tin+ + ne) at a sufficiently higher applied voltage coupled with the dissolution of Tin+ ions in the electrolyte to form localized pits (a compact Ti oxide), and (iii) achieving a balance between Tin+ solvation and oxide formation in order to form the nanotube structures. (38) Ti-metal plates were anodized at +42 V in an ethylene glycol (EG) electrolyte on solution composed of 0.25 wt % NH4F and 2 wt % H2O for 6 h. (39) After anodization, amorphous TNT was dip-coated in a 250 mM Co(NO3)2 ethanol solution for 1 min, pulled up at the rate of 10 mm/min, and dried at room temperature for 2 min. These processes were repeated three times. Bare- and Co-loaded-amorphous TNT were annealed at 450 °C in air for 1 h to make bare- and Co-TNT, respectively. To form Black TNT and Co-Black TNT, the bare- and Co-loaded amorphous TNTs were annealed at 450 °C in a stream of 5% H2/Ar for 30 min. A higher level Co-doping of Black TNT (containing, i.e., Co-Black TNT/CoOx) was prepared by drop-casting 125, 250, and 500 mM of the Co(NO3)2/ethanol solution. The Co loading was determined to be 0.54 μmol/cm2 for Co-Black TNT and 3.12, 4.20, and 7.11 μmol/cm2 for Co-Black TNT/CoOx, using inductively coupled plasma mass spectrometry (ICP-MS). Four samples (Co3O4, Co3O4/rGO, Co-TiO2, and CoFe2O4) were prepared for comparison with Co-Black TNT (see experimental details in the Supporting Information for information regarding the preparation of four control samples).

Characterization

The crystalline phase was identified via X-ray diffraction (XRD) (PANalytical X’Pert Pro). The surface chemical composition was analyzed using X-ray photoelectron spectroscopy (XPS, Surface Science M-Probe ESCA/XPS). Scanning electron microscopy (SEM) (Zeiss, Model 1550VP) coupled with energy-dispersive X-ray spectroscopy (EDX) were used to obtain SEM and EDX images. The surface charge of Co-Black TNT was analyzed using an electrophoretic light scattering spectrophotometer (ELSZ-1000).

Experimental Procedures and Analyses

The prepared TNT electrodes were dipped into distilled water. An aliquot of the organic substrate stock solution and the PMS stock solution was added into a magnetically stirred reactor. The solution was typically buffered at pH 7.2 using phosphate buffer pair (3 vol %). The 4-CP removal activity was not changed in unbuffered solution, which demonstrates the marginal effect of phosphate buffer (Figure S1 in the Supporting Information) in the Co-Black TNT/PMS reaction system. To evaluate pH effects, the experimental solution was unbuffered, and the initial pH values (3, 5, 9, and 11) were established using a standard solution of either HClO4 or NaOH. Sample aliquots versus time were withdrawn from the reactor using a 1 mL pipet and injected into a 2 mL amber glass vial containing excess methanol (0.1 M) to inhibit the reaction of any residual radicals.
The concentrations of the organic substrates were determined using a high-performance liquid chromatography (HPLC) system (Agilent 1100 series) that was equipped with a Zorbax XDB column. The eluent consisted of a binary mobile phase of acetonitrile and formic acid (10%:90% for phenol, 4-CP, and BA and 30%:70% for BPA). Chloride ion concentrations produced via dechlorination of 4-CP were monitored using an ion chromatograph (IC, Dionex, USA) with an anion-exchange column (Ionpac AS 19). The removal of total organic carbon (TOC) was determined using a TOC analyzer (Aurora TOC). PMS concentrations were spectrophotometrically determined by following a standard method. (40) For electron paramagnetic resonance (EPR) analyses, 5-tert-butoxycarbonyl 5-methyl-1-pyrroline-N-oxide (BMPO) was used as a spin-trapping agent for SO4 –. The EPR spectra were recorded on a Bruker EMX X-band CW-EPR spectrometer using the following conditions: microwave power = 20 mW, microwave frequency = 9.836, and at room temperature. The concentration of Co leached in the solution was determined by ICP-MS.

Electrochemical Measurements

Electrochemical impedance spectroscopy (EIS) and Mott–Schottky plots were measured using a Biologic VSP-300 potentiostat. TNT electrodes, stainless steel, and Ag/AgCl were utilized as the working electrode, a counter electrode, and a reference electrode, respectively. The electrodes were immersed in 0.1 M NaClO4 solution. The EIS plots were obtained over the frequency range of 200 kHz to 0.1 Hz. Mott–Schottky analysis was performed using a potential range from −1.0 V to 1.0 V (vs Ag/AgCl) at a frequency of 200 kHz and alternating current (AC) voltage of 25 mV.

Results and Discussion

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Physicochemical Properties of Synthesized TNTs

The product TNT samples were characterized by using XRD and SEM and the distribution of elements within Co-Black TNT was analyzed by EDX (see Figures S2–S4 in the Supporting Information and accompanying discussion in the Supporting Information for details).
The oxidation states and oxygen vacancies were analyzed by XPS spectra (Figure 1). The two peaks corresponding to Co 2p3/2 and 2p1/2, centered at 781 and 797 eV, respectively, were confirmed in the Co 2p XPS spectra of Co-doped TNTs, (41) and they were shifted to a higher binding energy, compared to the spectra of Co3O4 (Figure 1a). This result indicates that Co2+ only is present in Co-doped TNTs, whereas the mixed states of Co2+ and Co3+ are contained in Co3O4. (42) The both Co 2p peaks were not observed in bare- and Black-TNT (data not shown). We analyzed the XPS spectra of O 1s to confirm the oxygen vacancies in the prepared samples. The O 1s XPS spectra were deconvoluted with three major peaks, located at 529.9, 531.8, and 532.6 eV (Figure 1b), which correspond to lattice oxygen species (O2), adsorbed oxygen (e.g., O22– and O), and hydroxyl groups (OH), respectively. (43,44) The peak at 531.8 eV is closely related to the oxygen vacancies, since the molecular oxygen is dissociatively adsorbed on surficial oxygen vacancies. (41,42) The relative concentrations of the oxygen vacancies were estimated indirectly, using the peak at 531.8 eV. Co-Black TNT (25%) was found to have a higher relative concentration of oxygen vacancies, compared to Black TNT (21%), Co-TNT (17%), and bare TNT (11%). The Ti 2p XPS measurement (Figure 1c) confirmed the presence of Ti3+ on the surface of doped TNTs. The typical two peaks were observed at binding energies of 464.8 (for Ti 2p1/2) and 458.9 eV (for Ti 2p3/2) in bare- and Co-TNT, (45) whereas they were shifted to a lower binding energy in hydrogenated TNTs (Black and Co-Black TNT), because of a partial reduction of Ti4+ to Ti3+.

Figure 1

Figure 1. XPS spectra of (a) Co 2p, (b) O 1s, and (c) Ti 2p signals in Co-Black TNT, Co-TNT, Black TNT, bare TNT.

The electrochemical impedance spectroscopy (EIS) was used to evaluate the charge-transfer resistances of synthesized TNTs. A smaller arc size in a EIS Nyquist plot correlates to a smaller resistance to charge transfer on the surface of electrode. (46) The arc size in EIS Nyquist plots decreased in the following order: bare TNT > Co-TNT > Black TNT > Co-Black TNT (see Figure 2a). This result suggests that the electron transfer in the Co-Black TNT is more efficient than that for the other TNTs. Electron transfer efficiency is confirmed by Mott–Schottky analysis, as shown in Figure 2b. The Co-Black TNT gave a flat slope, compared to the other TNTs. This behavior also indicates enhanced electron transport. (35) The presence of oxygen vacancies and Ti3+ as donor states below conduction band of Co-Black TNT results in a redistribution of excess electrons between the nearest neighboring Ti atoms and oxygen vacancy sites to achieve charge balance, which, in turn, increases the charge carrier density. (47) As a result, the electrical conductivity of Co-Black TNT is enhanced.

Figure 2

Figure 2. (a) Nyquist plots of TNT, Co-TNT, Black TNT, and Co-Black TNT. Nyquist plots were obtained in the frequency range of 200 kHz to 0.1 Hz. (b) Mott–Schottky plots in electrochemical impedance spectroscopy. The Mott–Schottky plots were measured at a fixed frequency of 200 kHz in aqueous NaClO4 solution (0.1 M).

PMS Activation by TNTs

The Co-Black TNT electrodes had a higher efficiency for 4-CP degradation, stoichiometric Cl production, and TOC removal, compared to the other TNTs (e.g., bare-, Co-, and Black-TNT) (Figures 3a–c). However, in the absence of the catalyst (Co-Black TNT) or PMS, there was negligible 4-CP degradation (see Figure S5 in the Supporting Information). This implies that 4-CP is mainly degraded via PMS activation on the Co-Black TNT surface. 4-CP degradation by bare TNT was insignificant, because PMS cannot be activated in the absence of Co2+. Despite the lack of Co2+ in Black TNT, 49% of the 4-CP was decomposed over the same time frame. This result may be ascribed to that Black TNT with a high conductivity (see Figure 2) mediates a facial electron transfer from 4-CP to PMS (i.e., nonradical mechanism). The activation of PMS in Co-TNT markedly accelerated 4-CP degradation and it was further enhanced in Co-Black TNT, which is ascribed to superior properties of Co-Black TNT for PMS activation.

Figure 3

Figure 3. (a) Degradation of 4-CP and (b) production of chloride ions as a result of 4-CP dichlorination and (c) TOC removal after 1 h with TNT, Black TNT, Co-TNT, and Co-Black TNT in the presence of PMS. (c) 4-CP removal by various Co-based materials in the presence of PMS, (d) Oxidative degradation of various organic compounds by PMS activated on Co-Black TNT ([organic pollutants]0 = 100 μM; [PMS]0 = 1 mM; [phosphate buffer]0 = 3 vol %; pHi = 7.0).

PMS activation is initiated by the adsorption of PMS on the surface of heterogeneous catalysts (e.g., metal oxides) and the charge transfer between catalysts and PMS generates SO4 – that is subsequently desorbed from the catalyst surface into the aqueous solution for degrading organic pollutants. (48) In addition, a fast redox cycle (Mn+/Mn+1) of metal ions located on the catalyst surface enhances the generation of SO4 –. (48) Therefore, the heterogeneous catalysts should have a strong interaction with PMS and the organic substrates coupled with a high electrical conductivity that results in a fast redox cycling of key metal ions to efficiently activate PMS. Co-Black TNT contains a significant number of oxygen vacancies (see Figure 1b) that allow for the facile chemical bonding of PMS. (48) Oxygen vacancies, which are also efficient oxygen ion conductors, allow for the facile redox cycling of Co2+/Co3+ with PMS (see eqs 1 and 2). (48) In contrast, Co3O4 has a low oxygen ion conductivity, which limits its activity and stability. (49)(1)(2)where VO·· and OO× represent a doubly charged oxygen vacancy and the oxygen ion in an oxygen site on the Co-Black TNT surface, respectively. In addition, the presence of defect sites such as oxygen vacancies and Ti3+ enables a faster surface reaction via a higher electron transfer capability of Co-Black TNT, as demonstrated by the electrochemical analysis (see Figure 2). (34,35) As a result, the PMS activation efficiency of Co-Black TNT was much higher than that of other TNTs (see Figures 3a–c). However, the degradation of 4-CP was reduced with increasing Co concentration in the Co-Black TNT (Figure S6 in the Supporting Information). This result is attributed to an amorphous CoOx layer formed on the top of Co-Black TNT (Figure S7 in the Supporting Information) that blocks the open-channel structure of the nanotubes, effectively reducing the electrical conductivity (Figure S8 in the Supporting Information) and inhibiting the mass transfer of PMS and substrates. (32) The characteristic peaks of cobalt oxide were not observed in the XRD patterns of Co-Black TNT/CoOx (data not shown), which implies that the CoOx layer is amorphous. We further compared the 4-CP removal activity of Co-Black TNT with Co3O4, Co-TiO2, CoFe2O4, and Co3O4/rGO, as shown in Figure 3d. The 4-CP removal rate was the highest for Co-Black TNT, with a pseudo-first-order rate constant of 0.28 min–1. In comparison, the corresponding rate constants for the other cobalt materials were as follows: for Co3O4/rGO, k = 0.18 min–1; for CoFe2O4, k = 0.17 min–1; for Co3O4, k = 0.13 min–1; and for Co-TiO2, k = 0.11 min–1.
The Co-Black TNT/PMS system also had a relatively high efficiency for the degradation of phenol and bisphenol A, as shown in Figure 3e. In addition, benzoic acid was also degraded readily in the Co-Black TNT/PMS system.

The Mechanism of PMS Activation

To confirm the generation of SO4 – as the main oxidant derived from PMS activation on the TNTs, the kinetics of 4-CP degradation were determined in the presence of excess MeOH, which was used as a SO4• – scavenger (Figure 4a). The presence of excess MeOH significantly quenched the oxidation of 4-CP in Co-Black TNT, which demonstrates that 4-CP is mainly degraded by SO4 – generated from PMS activation. Several quinone species including hydroxyhydroquinone (HHQ), hydroquinone (HQ), and benzoquinone (BQ) were found as reaction intermediates during 4-CP oxidation in the Co-Black TNT/PMS system (Figure S9 in the Supporting Information). This result suggests that (i) 4-CP oxidation leads to the generation of chloride and intermediates via attack of SO4 – generated in Co-Black TNT/PMS system and (ii) SO4 – radical oxidation also results in the reduction of TOC, as shown in Figure 3c. However, the possible contribution of hydroxyl radical (OH), as quenched by methanol, cannot be ruled out as a contributor to overall oxidation. To confirm this, the quenching effect of MeOH was compared with that of t-BuOH in 4-CP degradation by the Co-Black TNT/PMS system. MeOH and t-BuOH have a similar bimolecular rate constant for reaction with OH (9.7 × 108 M–1 s–1 and 6.0 × 108 M–1 s–1, respectively), but the rate constant for MeOH + SO4• – (3.2 × 106 M–1 s–1) is much higher than that for t-BuOH + SO4 – (4.0 × 105 M–1 s–1). (13,50) Thus, MeOH and t-BuOH scavenge OH at similar rates, but MeOH is clearly more efficient than t-BuOH for SO4 – scavenging. As shown in Figure S10 in the Supporting Information, the 4-CP degradation was more effectively quenched by MeOH, compared to t-BuOH. This result indicates the OH is a minor contributor to the net degradation of 4-CP degradation in the Co-Black TNT/PMS system. In comparison, the pronounced quenching effects were not observed in the rate of 4-CP degradation on Black TNT. This suggests that 4-CP is mainly oxidized by electron transfer from 4-CP to PMS on the surface of the Black TNT surface. The radical-mediated oxidation processes can also be influenced by NOMs. (20,51,52) The effect of NOM was found to be similar to that of methanol on the degradation of 4-CP by TNTs (see Figure 4b). The 4-CP removal rate was markedly inhibited with different NOMs in the case of the Co-doped TNTs (Co-TNT and Co-Black TNT), while the degradation on Black TNT was not affected by NOM. This result further indicates different reaction pathways for PMS oxidation as catalyzed by Co-doped TNTs, compared to Black TNT. This result clearly shows the role SO4 – formation pathway on the Co-doped TNTs (i.e., the radical mechanism), compared to the direct electron transfer pathway for 4-CP to PMS (i.e., nonradical mechanism) on the Black TNT. The different PMS activation mechanisms between Co-Black TNT and Black TNT were further demonstrated by PMS decomposition in the presence and absence of 4-CP, as shown in Figure S11 in the Supporting Information. The effect of the electron donor (i.e., 4-CP) is minimized in the radical reaction pathway of Co-Black TNT, while PMS is not effectively decomposed in the absence of an electron donor in the nonradical pathway on Black TNT. (10) The generation of SO4 – was directly detected using the EPR spin-trapping technique (see Figure 4c). The peaks characteristic of the DMPO–SO4 – adduct appeared in the EPR spectrum of the Co-doped TNTs. However, the peaks corresponding to 5-tert-butoxycarbonyl-5-methyl-2-oxo-pyrroline-1-oxyl (BMPOX), as a product of direct BMPO oxidation, (53) was observed in the case of the bare TNTs and the Black TNTs. The schematic illustration in Figure 4d highlights the role of Co-Black TNT in the activation of PMS, leading to the formation of SO4 – as a principal oxidant for the degradation of 4-CP (i.e., the radical pathway). In contrast, the Black TNT results in 4-CP oxidation via direct electron transfer from 4-CP to PMS (i.e., the nonradical pathway).

Figure 4

Figure 4. Effect of (a) MeOH and (b) NOM on the rate of 4-CP degradation by PMS activated on TNTs except for bare TNT. (c) EPR spectra obtained in the aqueous BMPO and PMS with TNTs. (d) Schematic illustrations for radical pathway on Co-Black TNT (left) and nonradical pathway on Black TNT (right) for 4-CP degradation. (c) EPR spectra obtained the aqueous BMPO and PMS with TNTs (1 min after PMS activation). [4-CP]0 = 100 μM (for a); [MeOH]0 = 100 mM (for a); [NOM]0 = 10 ppm (for b); [BMPO]0 = 0.15 mM (for c); [PMS]0 = 1 mM; [phosphate buffer]0 = 3 vol %; pHi = 7.0).

Effect of Initial pH on the PMS Activation and Co Leaching

Figure 5 shows the rates of 4-CP degradation in PMS-activated system with Co-