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Hydroxylamine-assisted peroxymonosulfate activation using cobalt ferrite for sulfamethoxazole degradation
Journal Pre-proofs Hydroxylamine-assisted peroxymonosulfate activation using cobalt ferrite for sulfamethoxazole degradation Dasom Oh, Chung-Seop Lee, Yu-Gyeong Kang, Yoon-Seok Chang PII: DOI: Reference:
S1385-8947(19)33166-3 https://doi.org/10.1016/j.cej.2019.123751 CEJ 123751
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Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
6 August 2019 15 November 2019 6 December 2019
Please cite this article as: D. Oh, C-S. Lee, Y-G. Kang, Y-S. Chang, Hydroxylamine-assisted peroxymonosulfate activation using cobalt ferrite for sulfamethoxazole degradation, Chemical Engineering Journal (2019), doi: https:// doi.org/10.1016/j.cej.2019.123751
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Hydroxylamine-assisted peroxymonosulfate activation using cobalt ferrite for sulfamethoxazole degradation
Dasom Oh, Chung-Seop Lee, Yu-Gyeong Kang, Yoon-Seok Chang*
Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, 790-784, Republic of Korea
*Corresponding author. Tel: +82-54-279-2281. Fax: +82-54-279-8299. Email: [email protected]
Abstract
Hydroxylamine (HA), which is a reductant for the heterogeneous catalysts, was used to increase the activation of peroxymonosulfate (PMS) by cobalt ferrite (CoFe2O4) to degrade sulfamethoxazole (SMX). Here we added HA to increase the efficiency of Fe3+/PMS system. The CoFe2O4-PMS system with 0.1 g/L of CoFe2O4, 1 mM of PMS, and 0.1 mM of HA completely removed 20 mg/L of SMX at pH 5 in 120 min. This system showed higher SMX-degradation efficiency than other cobalt oxides and iron oxides. This increased SMX-degradation efficiency was attributed to (1) the inhibition of self-decomposed radicals and (2) the increased Fe3+/Fe2+ and Co2+/Co3+ redox cycles of CoFe2O4 due to the reducing power of HA. Additionally, CoFe2O4 exhibited superb reactivity during six successive degradation cycles when it was applied in a real water system. The degradation mechanism of SMX was assessed using liquid chromatography-mass spectrometry. This study provided a practical and novel strategy for the removal of pharmaceuticals and personal care products (PPCP).
Keywords: peroxymonosulfate, hydroxylamine, cobalt ferrite nanoparticles, PPCP, advanced oxidation process
1. Introduction The advanced oxidation process (AOP) based on sulfate radicals (SO 4― •) is used to degrade organic pollutants in water treatment systems.1,
2
SO 4― • has a high
standard redox potential (2.5 - 3.1V),3,5 and is therefore a more efficient oxidant than hydroxyl radical (•OH). Generally,
SO 4― • is generated by activation of
peroxymonosulfate (PMS) or persulfate (PS); PMS may be regenerated by a redox cycle, and therefore has a longer lifetime than PS.6 There have been extensive studies on the methods of activating PMS, such as UV7, heat8, transition metals9 and heterogeneous catalysts10. Although heat and UV are simple engineering methods to generate sulfate radicals from PMS, the disadvantage of high energy requirement restricts their field application. Among the transition metals, Co2+ is reported as the best PMS activator for SO 4― • production11 but the leached Co2+ can also be toxic, so its applicability for water remediation is constrained.12 Hence, this process requires a heterogeneous PMS activator that would be easily separated from water and has good stability and reactivity with low cobalt leaching. PMS activation with heterogeneous cobalt catalysts such as cobalt oxides13 and cobalt ferrites14,
40
might be a useful strategy. These heterogeneous catalysts have
better catalytic activity and lower cobalt dissolution; they are also magnetic, so they can be separated easily from the products which are non-magnetic. Spinel cobalt ferrite (CoFe2O4) has strong Co−Fe interaction, and therefore has excellent resistance to cobalt dissolution.15 The Fe3+ in CoFe2O4 can activate PMS, but electron transfer
from Fe3+ to PMS is thermodynamically less favorable than from Co2+,
18
so the
contribution of Fe3+ to PMS activation seems to be regligible.16, 17 To overcome this drawback, the introduction of hydroxylamine (HA) as a reductant into the Fe2+/PMS system considerably activates the Fe3+/Fe2+ cycle, thereby promoting the generation of reactive radicals and the degradation of organic compounds20, 39. However, the effect of HA on the mixed iron-cobalt catalyst/PMS system has not been quantified, yet. Here, we suggest an efficient heterogeneous PMS activation system that uses CoFe2O4, PMS and HA to degrade organic pollutants including antimicrobial agents such as sulfamethoxazole (SMX). SMX is widely used to prevent animal diseases and to promote the growth of livestock21, and is frequently found in the effluent of water treatment plants.22 Overuse of SMX could contaminate the water environment and cause health risks23. Therefore, the concentration of SMX along with the other antibiotics must be reduced efficiently in the water environment. In this study, we focused on (i) investigation of the function of HA in the PMS activation system, (ii) quantification of the effects of experimental factors on the SMX-removal efficiency of the CoFe2O4/PMS/HA system, (iii) characterization of the CoFe2O4 surface after introduction of HA, (iv) identification of the radical species, and (v) identification of the degradation products of SMX.
2. Materials and Methods 2.1. Materials Oxone (KHSO5∙0.5KHSO4∙0.5K2SO4, PMS), hydroxylamine (NH2OH, 99.9%),
sulfamethoxazole (SMX, 99.0%), cobalt chloride hexahydrate (CoCl2∙6H2O), ferrous chloride hexahydrate (FeCl3·6H2O), benzoquinone, hydrochloric acid (HCl), and 5,5dimethyl-1-pyrolin-N-oxide (DMPO) were of analytical grade and purchased from Sigma-Aldrich (U.S.A). Tert-butyl alcohol (TBA), methanol (MeOH), ethanol, and sodium hydroxide (NaOH) were of guaranteed reagent grade and supplied by Daejung Chemical Reagent Company (Korea).
2.2. Synthesis and characterization of CoFe2O4 CoFe2O4 was synthesized using a coprecipitation method modified from the literature24. FeCl3 (0.5 M) and CoCl2 (0.5 M) were mixed in distilled water (DW) with vigorous stirring at 60 °C. After the metal salts had clearly dissolved, NaOH solution in DW (1 M) was added until the pH reached 12. The precipitated layer was separated by centrifugation and washed with DW and ethanol, then dried in a vacuum oven for 12 h at 60 °C. The dried powder was calcined for 12 h at 300 °C in a furnace. The surface morphology of CoFe2O4 was observed using a JSM 7401F (JEOL) high-resolution scanning electronic microscope (HR-SEM). A transmission electron microscope (TEM) image and EDS mapping were obtained using a JEM-2100F (JEOL). The X-ray diffraction (XRD) results were recorded on a Rigaku D/Max-2500 using Cu Kα radiation. The magnetic hysteresis loops of CoFe2O4 were acquired using an MPMS XL-7 (Quantum Design) magnetic property measurement system (MPMS). The X-ray photoelectron spectroscopy (XPS) analysis was performed using an ESCALAB 250 (VG Scientific). The surface properties were obtained using a
Fourier infrared spectroscope (FT-IR, iS50R of Thermo Scientific).
2.3. Batch experiment Batch tests were performed in 80-mL glass vials capped with Teflon septa. A 5mg sample of catalyst was dropped into 50 mL of SMX solution in a standard experiment. An aqueous SMX standard solution of 1.58 mM was prepared, then diluted to 79 M. Then 1 mM of PMS and 0.1 mM of HA were added to the solution. Then 1 mM of PMS and 0.1 mM of HA were added to the solution with initial pH control using HCl and NaOH. The vials were placed on a rolling mixer during the test. At certain time intervals, 1 mL of the samples were withdrawn with a syringe and filtered through a 0.45- m filter. For the reusability test, the used catalyst was separated magnetically after 120 min. Then CoFe2O4 was dispersed in the solution under the same experimental conditions. All treatments were replicated three times.
2.4. Analytical Methods SMX analysis was performed using an Agilent LC/MS system (Agilent 1260 infinity with 6120 quadrupole), which consists of a Luna C-18(2) column, (150ⅹ4.6 mm, 5 m, Phenomenex), and a UV detector coupled with a mass spectrometer detector (MSD). The mobile phase composition was 60:40 (v/v) distilled water and methanol, and the flow rate was 1.0 mL min-1. Under the given conditions, the SMX was eluted at 5.8 min with a maximum absorbance at 265 nm to determine the concentration. In MS detection, the samples were analyzed in APCI-positive mode
electrospray ionization from 50 m/z to 1000 m/z. The analysis method for the concentrations of residual PMS and HA followed previous studies.41, 42 Electron spin resonance (ESR) experiments were performed to measure the reactive radicals with DMPO as a spin-trapping agent using ESR spectroscopy (JESX310, JEOL). The concentration of total organic carbon was obtained using a TOC-V (Shimadzu). The NH4+ , NO3― , and NO2― concentrations were determined using an ion chromatograph (DX-120, Dionex). Total metal concentrations were measured using inductively coupled plasma optical emission spectrometry (ICP-OES, iCAP 7400, Thermo).
3. Results and Discussion 3.1. Characterization of CoFe2O4 The morphology and particle size of synthesized CoFe2O4 were characterized using HR-TEM (Fig. 1a). Most of the particles were aggregated at micrometer scale; the calculated average size of each particle was 36 nm. The prepared particles had polycrystalline structures in the HR-TEM image. The EDX spectrum (Fig. 1a and 1b) indicated that the atomic Co:Fe ratio in CoFe2O4 was 25.0:48.6 which was close to the stoichiometric ratio of 1:2. The XRD pattern (Fig. 1c) of CoFe2O4 particles showed peaks at 2θ = 30.4°, 35.8°, 43.6°, 54.0°, 57.4° and 62.9°, which is corresponded to the Bragg planes (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0), respectively. The Scherrer equation τ=
Κλ 𝛽cos 𝜃
(1)
was applied to the main diffraction peak at 2θ = 35.8° to calculate that the average
crystallite size was 35 nm which was close to the morphological observations. The magnetization characteristics were confirmed using MPMS analysis (Fig. 1d); CoFe2O4 had saturation magnetization MS = 21.6 emu/g. The CoFe2O4 suspension in DW was dark brown, but in the presence of magnetization, it became clear (Fig. 1d, inset images) It means that the synthesized CoFe2O4 was able to separate from the water. The N2 adsorption-desorption by CoFe2O4 showed hysteresis (Fig. S1). The calculated BET area of CoFe2O4 was 23.08 m2/g. These results indicate that nanosized spinel CoFe2O4 ferrite crystals were synthesized, and that they had magnetic properties.
3.2. Catalytic efficiency of the CoFe2O4/PMS/HA system Addition of HA to the CoFe2O4/PMS system increased its SMX-removal effectively (Fig. 2a). In the CoFe2O4 system without HA, approximately 40% of SMX was decomposed within 120 min (k = 5.5ⅹ10-3 min-1); the slow degradation rate might be due to the low PMS activation efficiency of the Fe3+ in the CoFe2O4 structure. On the other hand, the addition of HA greatly increased the reactivity, and the CoFe2O4/PMS/HA system degraded most of the SMX within 120 min (k = 3.4ⅹ10-2 min-1). These results indicate that both CoFe2O4 and HA promote the activation of PMS removal of SMX in the CoFe2O4/PMS/HA system. When only CoFe2O4 was present, 12% of SMX was eliminated by adsorption to the surface of CoFe2O4. In addition, HA in the absence of PMS did not affect the degradation of SMX, but HA itself could activate PMS slightly. During the reaction, the residual PMS in the system was reduced to 40% of the initial concentration, but 15% of
reduced concentration is due to natural decrease, such as adsorption and dissolution of PMS, which are not related to the contaminants degradation (Fig. S12). Plus, residual HA concentration was measured indirectly by the addition of acetone. After the reaction, about 40% was decomposed, showing no selectivity for the target contaminant SMX (Fig. S13). The composition of cobalt ferrite was investigated to affect the SMX removal with PMS and HA (Fig. 2b). The CoFe2O4 with HA showed a faster reaction rate than that of Co0.5Fe2.5O4 (k = 1.7ⅹ10-2 min-1). However, the rate of SMX degradation by Fe3O4 (k = 2.2ⅹ10-2 min-1) was still slower than by CoFe2O4. This result shows that Co2+ in spinel ferrite increases its ability to activate PMS. Furthermore, Fe3O4 has a reaction rate that is similar to the reaction rate of Fe2O3 which has only Fe3+ as an iron source (k = 2.2ⅹ10-2 min-1). Based on the results, the increase in SMX removal by HA is more pronounced with Fe3+ than with Co2+. This difference was obvious when compared with experiments that did not include HA (Fig. S2). To select target compounds for this study, we compared the rates at which various organic pollutants were degraded. Within 120 min, the CoFe2O4/PMS/HA system degraded 46% of BPA, 65% of CBZ, 100% of SMX and 100% of DCF (Fig. S3a). DCF was well removed even without the addition of HA, so SMX was used as a model compound (Fig. S3b). The catalytic activity the CoFe2O4/PMS/HA system increased as the calcination temperature decreased; calcination of CoFe2O4 at 150 °C was the ideal experimental condition for PMS activation (Fig S4). However, calcination at higher
temperatures reduced the cobalt elution26, so the experiments were conducted using the CoFe2O4 particles that had been calcined at 300 °C.
3.3. Effects of operating parameters for the CoFe2O4/PMS/HA system Various experimental conditions affected the catalytic activity of the CoFe2O4/PMS/HA system (Fig. 3). With the constant initial PMS concentration (79 μM), the degradation of SMX by the CoFe2O4/PMS/HA system was dependent on the CoFe2O4 concentration (Fig. 3a). SMX was completely degraded at fixed [CoFe2O4] = 100 mg/L, but only 75.3% was degraded at the SMX [CoFe2O4] = 10 mg/L. As [CoFe2O4] increased, the active sites on the surface also increased, so the PMS activation and SMX removal rates increased. The PMS concentration also affected the SMX degradation in the CoFe2O4/PMS/HA process (Fig. 3b). Increase in the PMS concentration from 0.2 to 2.0 mM caused increase in the SMX degradation kinetics. The linear correlation between the PMS concentration and the reaction rate would be a result of the number of radicals generated by PMS. The efficiency of SMX degradation increased as the HA concentration increased from 0.1 to 1.0 mM, but the rate of increase in degradation rate diminished at [HA] >2.0 mM. (Fig. 3c). These results indicated that HA could exist in the solution as a reducing agent at concentrations < 1.0 mM, but not at higher concentrations, due to the competition with SMX.25
pH, also was shown to affect the SMX degradation in the CoFe2O4/PMS/HA process (Fig. 3d). When the initial pH of the solution was approximately 3, the SMX removal was significant in the CoFe2O4/PMS/HA process due to the dissolution of metal cations into the solution.26. As the pH increased from 3 to 9, the reaction rates decreased from 3.5ⅹ10-2 min-1 to 2.3ⅹ10-2 min-1 because the pH affected the speciation of HA (NH3OH+, NH2OH, and NH2O-), which influenced the efficiency of the electron transfer27. Furthermore, as the pH of the solution increased, the selfdecomposition of PMS increased, so the efficiency of SMX degradation decreased.28
3.4. Metal leaching effects of the CoFe2O4/PMS/HA system To
evaluate
the
homogeneity/heterogeneity
of
the
reaction
in
the
CoFe2O4/PMS/HA system, the concentration of leached metal ions was determined after reactions (Fig. 4a). It was shown that Co was leached more than Fe, although the CoFe2O4 structure has two times of Fe than Co. After 120 min, the concentration of leached Fe was 0.15 mg L−1 which was only 0.09–0.21% of the total Fe in the system, whereas the concentration of leached Co was 0.20 mg L-1 which was not much different from a previous toxicity study of CoFe2O4.29, 30 Based on the leached metal conditions, we compared the contribution of radical reactions occurring on the surface and the reaction in the solution. The eluate, in which CoFe2O4 was leached in D.I. water for 2 hours, was used with HA and PMS. After the reaction in a homogeneous system, around 75% of SMX was degraded compared to the adsorption and heterogeneous degradation (Fig. 4b). This result
indicated that the effects of the leached iron and cobalt were greater than the reaction on the surface.
3.5. Surface transformation of the CoFe2O4/PMS/HA system Elemental composition of the surface of CoFe2O4 with HA was investigated using XPS. The full scan survey of the samples presented Fe, Co, O, and C peaks (Fig. S5). Detailed XPS data (Fig. 5a) of the Fe 2p level peak showed a main peak at ~ 710.2 eV which corresponded to CoFe2O4, and binding energies of 711.5 eV and 709.0 eV that were assigned to Fe3+ and Fe2+, respectively.31,
32
The reaction without HA caused
surface oxidation, so the binding energy increased compared to the material before the reaction. In the reaction with HA, the binding energy did not change, suggesting that surface reductions might be occurred. The Co 2p XPS spectrum (Fig. 5b) suggests that the spectrum is relevant to the CoFe2O4 samples and indicates similar trends with the Fe 2p level after the reaction. The areas of the Co 2p and Fe 2p peaks were calculated and used to estimate the ratios of the cation concentration of CoFe2O4 (Table S1). The results demonstrated that Fe3+ and Co2+ were definitely transformed to Fe2+ and Co3+ with HA, whereas the ratios of Fe3+ and Co2+ increased in the system without HA. This result indicates that HA promoted the electron transfer in the Fe3+/Fe2+ and Co2+/Co3+ cycles. FT-IR was used to further assess the surface of CoFe2O4 after reaction in the CoFe2O4/PMS/HA system (Fig. S6b). In the scope of 580 cm−1 wavenumber, the CoFe2O4 displayed distinct peaks of Fe-O. In addition, the CoFe2O4 particles after the
reaction with HA exhibited a relatively weak Fe-O peak. After the reaction, the 3405 cm-1 band, which coincides with hydroxyl groups, appeared because of the presence of water. In addition, the clarity of the baseline confirmed that the HA and degradation products did not adsorb on the surface of CoFe2O4 during the reaction. The EDX results (Fig. S7) of CoFe2O4 after the reaction in the CoFe2O4/PMS/HA system showed that the atomic ratio of Co:Fe in the particle was 1:0.33, which meant that more Co than Fe was leached, as interpreted from Fig. 4. The FT-IR spectra and XPS results confirmed that the nitrogen from HA did not adsorb on the surface of CoFe2O4.
3.6. Identification of reactive species Two radical scavengers of methanol (MeOH) and tert-butanol (TBA) were used to distinguish the active radical species in the CoFe2O4/PMS/HA system. The scavengers were mixed with the solution at a sufficient concentration (1 M) to ensure preference between the radicals and the scavengers. The reaction rate constants between the radicals with MeOH (kSO4•− = 3.2 ⅹ106 M-1 s-1; k∙OH = 9.7 ⅹ 108 M-1 s-1) and TBA (kSO4•− = 4.0 ⅹ105 M-1 s-1; k∙OH = 6 ⅹ 108 M-1 s-1) were different33, so the functions of SO 4― • and •OH could be distinguished. The SMX-removal efficiency was reduced from 99.0% to 56.7% when MeOH was added, and to 75.7% when TBA was added (Fig. 6a). This result indicated that both SO 4― • and •OH occurred in the CoFe2O4/PMS/HA system but that SO 4― • contributed more to the degradation of SMX than •OH. In the system without HA, the degradation efficiency of SMX
decreased from 45.4% to 31.7% when MeOH was added, and to 42.3% when TBA was added (Fig. S8). This result means that •OH seems to have no significant effect on the PMS activation if HA is not present. In other words, the amount of •OH increases when HA is added in the CoFe2O4/PMS system. Hence, the addition of HA aids electron transfer of the radical pathways in the system and indirectly promotes the generation of SO4 and OH radicals To test this hypothesis, the active radical species in the system were detected using EPR. The radicals were captured using DMPO which is a spin-trapping agent. Overlapping DMPO-SO4 and DMPO-OH signals appeared in the CoFe2O4/PMS/HA system (Fig. 6b). These signals indicated that both SO 4― • and •OH existed in the PMS aqueous solution, confirmed by Fig. 6a. However, without HA, the DMPO-SO4 and DMPO-OH signals were not detected until 5 min after the addition of CoFe2O4. These DMPO-SO4 and DMPO-OH could be attributed to the multiplied •OH radicals as: 4,20,34
HSO5- + H2O ↔ H2O2 + HSO4-
(2)
H2O2 + Fe2+ → Fe3+ + •OH + OH-
(3)
When HA was added to the solution, the DMPOX signal was disappeared; this change indicates that the excessive •OH radicals that oxidized DMPO to DMPO-SO4 and DMPO-OH were easily removed by the regenerated Co2+: Co2+ + HSO5- → Co3+ + OH- + SO4-• .
(4)
The results of the radical-scavenger experiments demonstrate that the • OH radicals did not affect the degradation efficiency in the system when HA was not
present. Therefore, the excessive •OH radicals generated in the system without HA could not affect the pollutant degradation.
3.7. Determination of degradation intermediates According to the TOC removal data (Fig. S10b), the mineralization efficiency of SMX was lower than the total SMX removal amount, indicating that the mineralization was incomplete as a result of the formation of intermediate products. To propose the degradation pathway (Fig. 7), LC-MS experiments were conducted (Fig. S9) showing eight intermediate peaks [M + H]+ at m/z = 254, 185, 173, 143, 100, 99, 60 and 59. The structures of these intermediates were suggested by examining the MS fragmentation patterns, in which the degradation pathway from SMX to small organic compounds was hypothesized (Fig. 7). First, the S-N bond of SMX was cleaved, followed by dissociation of the other bonds and modification of the functional groups of the benzene and isoxazole rings, depending on the reactive radicals present. IC was also used to quantify the NH 4+ , NO 3― , and NO 2― that the reaction generated (Fig. S10a). NH 4+ and NO 3― , were produced rapidly within 60 min from the degradation products of SMX and HA. The concentration of NO 3― did not change after 60 min, but the concentration of NH 4+ gradually decreased as a result of adsorption. The amount of NO 2― was insignificant during the degradation process compared to the NH 4+ and NO 3― , because NO 2― is easily oxidized to NO 3― .
3.8. Reaction mechanism of the CoFe2O4/PMS/HA system From the surface, water molecules are adsorbed to the ≡Co and ≡Fe sites to produce ≡ Co2+ − −OH from ≡ Co(OH)+ and ≡ Fe3+ − −OH
36,37.
When PMS was
dropped into the solution, the ≡Co2+ and ≡Fe3+ of the surface reacted with PMS to generate •OH (Eqs. 5, 6). Then the ≡Co2+ − −OH on the CoFe2O4 surface activated PMS to generate bound SO 4― • and •OH (Eq. 7), and more ≡ Co2+ − −OH were generated (Eq. 8). Similarly, the ≡Fe2+ − −OH and ≡Fe3+ − −OH could also generate the surface-bound SO 4― • radicals from HSO5- (Eqs. 9, 10). The reported standard reduction potential of Co 3+ /Co 2+ redox pair is 1.81 V and that for Fe 3+ /Fe 2+ is 1.51 V, whereas the reduction reaction of HA is -1.87 V (Eq.11) 35. This comparison reveals that the regeneration of Co2+ and Fe2+ on the surface is thermodynamically favorable (Eq. 12). Furthermore, the regenerated ≡Co2+ decreased the excessive •OH radicals that might contribute to the self-decomposition of PMS, as ESR results indicated (Eq. 4). The SMX in the aqueous solution unceasingly reacted on the surface of CoFe2O4 and was degraded by SO4•− and •OH (Eq. 13). ≡ Co2+ + HSO5−→ ≡Co3+ + SO42−+ •OH
(5)
≡ Fe2+ + HSO5−→ ≡Fe3+ + SO42−+ •OH
(6)
≡ Co2+ − −OH + HSO5−→ ≡Co3+ − −OH + SO 4― • + OH−
(7)
≡ Co3+ − −OH + HSO5−→ ≡Co2+ − −OH + SO 4― • + H+
(8)
≡ Fe2+ − −OH + HSO5−→ ≡Fe3+ − −OH + SO 4― •
(9)
≡ Fe3+ − −OH + HSO5−→ ≡Fe2+ − −OH + SO 4― •+ H+
(10)
≡Fe3+/≡ Co3+ ≡Fe2+/≡ Co2+
(11)
2NH3OH+ → N2 + 2H2O + 4H+ + 2e-
(12)
SO 4― • (or •OH) + Organic pollutants →...→ CO2 + H2O
(13)
From the above reactions, we could hypothesize overall reaction mechanism of the SMX degradation in the CoFe2O4/PMS/HA system(Fig. 8).
3.9. Feasibility and reusability of the CoFe2O4/PMS/HA system Three water samples (Table S2) were used to test the integrated effects of the background on the degradation of SMX (Fig. 9a). For wastewater sample, the efficiency of SMX degradation was 87.5%, which was lower than the experiment in the distilled water because the chloride anions inhibited the PMS activation.38 In the groundwater sample #1, the efficiency of SMX degradation was similar to that with the distilled water in the HA containing system and only 42% without HA. However, in groundwater sample #2 the efficiency of SMX degradation was 98% with HA, which was not much different from the degradation efficiency of the system without HA (Fig. S11); this similarity might be the result of the high redox condition of the groundwater sample #2 (Eh = -132.3). In order to mimic the real environment, an additional experiment with a reduced initial SMX concentration (4 μM) was conducted. The degradation rate was slower than the control, however SMX was perfectly removed after 120 min. Six cycles of the same CoFe2O4 material were performed to estimate the reusability with the addition of HA. The efficiencies of SMX removal after 120 min
remained as high as 86.5% after six cycle runs, whereas the removal efficiency with Fe0 decreased to 53.5% (Fig. 9b). This result indicated that HA could help to maintain the reactivity CoFe2O4 against oxidation of its active catalytic sites (≡Co3+ and ≡Fe3+).
4. Conclusion
A novel CoFe2O4/PMS/HA system to effectively activate PMS for SMX degradation was devised. This system, which used environmentally benign reductant, degrades SMX much better than the conventional CoFe2O4/PMS system. The presented system was optimized for different pH values and concentrations of PMS, HA and NPs. Compared with conventional AOPs, which shows the optimum condition only in acidic pH, the CoFe2O4/PMS/HA system showed broad pH range to be applied even in neutral pH condition. Addition of HA to the CoFe2O4/PMS system significantly promoted the Fe3+/Fe2+ and Co2+/Co3+ cycles on the surface of CoFe2O4 by thermodynamically-favorable reaction with HA. Especially, HA reduced the selfdecomposition of the •OH radicals generated by Co2+/PMS. In other words, this system could maximize the efficiency of sulfate radical and minimize the disadvantage of cobalt based PMS activation. Furthermore, LC/MS analysis and IC were measured to provide detail degradation mechanisms of SMX. The LC/MS analysis suggested that most of the intermediates were generated by cleavage of S-N bonds, and IC results indicated trace amount of ionic byproducts. The proposed system showed good feasibility in actual water samples and maintained high
reactivity during reusability tests, which means the system could be applied in real remediation system.
Acknowledgement This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. 2017R1A2B3012681).
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Liang,
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Mohanty,
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Kurakalva,
A
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a)
b)
Fe
Co Fe Fe Co O
Co
0
c)
d)
(311)
(220) (511)
(400)
Magnetization (emu/g)
Intensity
CoFe2O4
(440)
(422)
5
10
15
20
20 CoFe2O4 10
0
-10
-20 20
30
40
50
2 theta (degree)
60
70
-10
-5
0
5
10
Magnetic Field (kOe)
Fig. 1. Characterization of the fabricated spinel CoFe2O4. a), b) HR-TEM images with EDX spectrum, c) XRD pattern, and d) magnetic hysteresis loop. Embedded image: CoFe2O4 suspension under no magnetization (left) and under magnetization (right).
a) 1.0
C/C 0
0.8
0.6 CoFe2O4 PMS+CoFe2O4
0.4
PMS+HA+CoFe2O4 Control HA+PMS HA+CoFe2O4
0.2
0.0 0
20
40
60
80
100
120
Time (min)
b) 1.0 Co0.5Fe2.5O4 CoFe2O4
0.8
Fe3O4
C/C 0
Fe2O3 Co3O4
0.6
0.4
0.2
0.0 0
20
40
60
80
100
120
Time (min)
Fig. 2. a) Degradation efficiency of sulfamethoxazole in CoFe2O4/PMS/HA system, and b) SMX degradation with various Co-Fe mixed oxides. Experimental conditions: [Pollutants]0 = 79.0 μM, [Oxides] = 0.1 g/L, [PMS]0 = 1 mM, [HA]0 = 0.1 mM, and pH = 5.
a)
b) 1.0
1.0
0.8 0 mg/L 10 mg/L 50 mg/L 100 mg/L 200 mg/L
0.6
0.4
C/C 0
C/C 0
0.8
0.2 mM 0.5 mM 1 mM 2 mM
0.6
0.4
0.2
0.2
0.0
0.0 0
20
40
60
80
100
120
0
20
40
Time (min)
80
100
120
d)
c) 0 mM 0.02 mM 0.05 mM 0.1 mM 0.2 mM 0.5 mM
1.0
0.8
0.6
1.0
0.6
0.4
0.4
0.2
0.2
0.0
pH=3 pH=5 pH=7 pH=9
0.8
C/C 0
C/C 0
60
Time (min)
0.0 0
20
40
60
Time (min)
80
100
120
0
20
40
60
80
100
120
Time (min)
Fig. 3. Effects of a) particle dosage, b) PMS concentration, c) hydroxylamine concentration, and d) pH on the degradation of SMX in CoFe2O4/PMS/HA system. Experimental conditions: [SMX]0 = 79.0 μM, [CoFe2O4] = 0.1 g/L, [PMS]0 = 1 mM, [HA]0 = 0.1 mM, and pH = 5.
a)
b) 1.0
C/C0
0.8 Adsorption Homogeneous PMS+HA+CoFe2O4
0.6
0.4
0.2
0.0 0
20
40
60
80
100
120
Time (min)
Fig. 4. a) Total leached metal concentrations in 120 min., b) Degradation of sulfamethoxazole in homogenous condition after CoFe2O4 is leached out.
Experimental conditions: [SMX]0 = 79.0
μM, [CoFe2O4] = 0.1 g/L, [PMS]0 = 1 mM, [HA]0 = 0.1 mM, and pH = 5.
a)
Fe 2p
Before reaction
Fe3+
Fe2+
Intensity
Without HA
With HA
735
730
725
720
715
710
705
Binding Energy (eV)
Co 2p
b)
Co2+
Intensity
Before reaction
Co3+
Without HA
With HA
810
805
800
795
790
785
780
775
Binding Energy (eV)
Fig. 5. XPS spectra of the a) Fe 2p and b) Co 2p regions for fresh CoFe2O4 samples and treated CoFe2O4 samples with or without hydroxylamine.
a) MeOH 1 M MeOH 2 M TBA 1 M TBA 2 M BQ 1 M Control
1.0
C/C 0
0.8
0.6
0.4
0.2
0.0 0
20
40
60
80
100
120
Time (min)
b)
Control
Intensity
Without HA
With HA
332
334
336
338
Magnetic field (mT)
Fig. 6. a) Effect of radical scavengers on SMX degradation, and b) ESR spectrum in a CoFe2O4/PMS/HA system with a DMPO (0.05 M) spin-trap. Experimental conditions: [SMX]0 = 79.0 μM, [CoFe2O4] = 0.1 g/L, [PMS]0 = 1 mM, [HA]0 = 0.1 mM, and pH = 5.
Fig. 7. Proposed degradation pathway of SMX in a CoFe2O4/PMS/HA system.
Fig. 8. Proposed reaction mechanism for the SMX degradation by a CoFe2O4/PMS/HA system.
a) 1.0
Wastewater Groundwater #1 Groundwater #2 Groundwater #2 (4 m SMX) Distilled water
C/C 0
0.8
0.6
0.4
0.2
0.0 0
20
40
60
80
100
120
Time (min)
b)
1.0 0.8
C/C 0
0.6 0.4 0.2 Fe0+PMS+HA CoFe2O4+PMS+HA
0.0
0
200
400
600
Time (min)
Fig. 9. a) Feasibility test in actual water body with spiked SMX and b) reusability test for CoFe2O4/PMS/HA system during six cycles. Experimental conditions: [SMX]0 = 79.0 μM, [CoFe2O4] = 0.1 g/L, [PMS]0 = 1 mM, [HA]0 = 0.1 mM, and pH = 5.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
There are no potential competing interests.
Highlights Hydroxylamine(HA) accelerated peroxymonosulfate(PMS) activation with CoFe2O4. Co(II)/Fe(III) cycle on the surface of CoFe2O4 was promoted by HA. Sulfate radical was major reactive substance in CoFe2O4/PMS/HA syste m. Pathway of sulfamethoxazole degradation was investigated using LC-MS and IC.
S1385-8947(19)33166-3 https://doi.org/10.1016/j.cej.2019.123751 CEJ 123751
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
6 August 2019 15 November 2019 6 December 2019
Please cite this article as: D. Oh, C-S. Lee, Y-G. Kang, Y-S. Chang, Hydroxylamine-assisted peroxymonosulfate activation using cobalt ferrite for sulfamethoxazole degradation, Chemical Engineering Journal (2019), doi: https:// doi.org/10.1016/j.cej.2019.123751
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© 2019 Elsevier B.V. All rights reserved.
Hydroxylamine-assisted peroxymonosulfate activation using cobalt ferrite for sulfamethoxazole degradation
Dasom Oh, Chung-Seop Lee, Yu-Gyeong Kang, Yoon-Seok Chang*
Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, 790-784, Republic of Korea
*Corresponding author. Tel: +82-54-279-2281. Fax: +82-54-279-8299. Email: [email protected]
Abstract
Hydroxylamine (HA), which is a reductant for the heterogeneous catalysts, was used to increase the activation of peroxymonosulfate (PMS) by cobalt ferrite (CoFe2O4) to degrade sulfamethoxazole (SMX). Here we added HA to increase the efficiency of Fe3+/PMS system. The CoFe2O4-PMS system with 0.1 g/L of CoFe2O4, 1 mM of PMS, and 0.1 mM of HA completely removed 20 mg/L of SMX at pH 5 in 120 min. This system showed higher SMX-degradation efficiency than other cobalt oxides and iron oxides. This increased SMX-degradation efficiency was attributed to (1) the inhibition of self-decomposed radicals and (2) the increased Fe3+/Fe2+ and Co2+/Co3+ redox cycles of CoFe2O4 due to the reducing power of HA. Additionally, CoFe2O4 exhibited superb reactivity during six successive degradation cycles when it was applied in a real water system. The degradation mechanism of SMX was assessed using liquid chromatography-mass spectrometry. This study provided a practical and novel strategy for the removal of pharmaceuticals and personal care products (PPCP).
Keywords: peroxymonosulfate, hydroxylamine, cobalt ferrite nanoparticles, PPCP, advanced oxidation process
1. Introduction The advanced oxidation process (AOP) based on sulfate radicals (SO 4― •) is used to degrade organic pollutants in water treatment systems.1,
2
SO 4― • has a high
standard redox potential (2.5 - 3.1V),3,5 and is therefore a more efficient oxidant than hydroxyl radical (•OH). Generally,
SO 4― • is generated by activation of
peroxymonosulfate (PMS) or persulfate (PS); PMS may be regenerated by a redox cycle, and therefore has a longer lifetime than PS.6 There have been extensive studies on the methods of activating PMS, such as UV7, heat8, transition metals9 and heterogeneous catalysts10. Although heat and UV are simple engineering methods to generate sulfate radicals from PMS, the disadvantage of high energy requirement restricts their field application. Among the transition metals, Co2+ is reported as the best PMS activator for SO 4― • production11 but the leached Co2+ can also be toxic, so its applicability for water remediation is constrained.12 Hence, this process requires a heterogeneous PMS activator that would be easily separated from water and has good stability and reactivity with low cobalt leaching. PMS activation with heterogeneous cobalt catalysts such as cobalt oxides13 and cobalt ferrites14,
40
might be a useful strategy. These heterogeneous catalysts have
better catalytic activity and lower cobalt dissolution; they are also magnetic, so they can be separated easily from the products which are non-magnetic. Spinel cobalt ferrite (CoFe2O4) has strong Co−Fe interaction, and therefore has excellent resistance to cobalt dissolution.15 The Fe3+ in CoFe2O4 can activate PMS, but electron transfer
from Fe3+ to PMS is thermodynamically less favorable than from Co2+,
18
so the
contribution of Fe3+ to PMS activation seems to be regligible.16, 17 To overcome this drawback, the introduction of hydroxylamine (HA) as a reductant into the Fe2+/PMS system considerably activates the Fe3+/Fe2+ cycle, thereby promoting the generation of reactive radicals and the degradation of organic compounds20, 39. However, the effect of HA on the mixed iron-cobalt catalyst/PMS system has not been quantified, yet. Here, we suggest an efficient heterogeneous PMS activation system that uses CoFe2O4, PMS and HA to degrade organic pollutants including antimicrobial agents such as sulfamethoxazole (SMX). SMX is widely used to prevent animal diseases and to promote the growth of livestock21, and is frequently found in the effluent of water treatment plants.22 Overuse of SMX could contaminate the water environment and cause health risks23. Therefore, the concentration of SMX along with the other antibiotics must be reduced efficiently in the water environment. In this study, we focused on (i) investigation of the function of HA in the PMS activation system, (ii) quantification of the effects of experimental factors on the SMX-removal efficiency of the CoFe2O4/PMS/HA system, (iii) characterization of the CoFe2O4 surface after introduction of HA, (iv) identification of the radical species, and (v) identification of the degradation products of SMX.
2. Materials and Methods 2.1. Materials Oxone (KHSO5∙0.5KHSO4∙0.5K2SO4, PMS), hydroxylamine (NH2OH, 99.9%),
sulfamethoxazole (SMX, 99.0%), cobalt chloride hexahydrate (CoCl2∙6H2O), ferrous chloride hexahydrate (FeCl3·6H2O), benzoquinone, hydrochloric acid (HCl), and 5,5dimethyl-1-pyrolin-N-oxide (DMPO) were of analytical grade and purchased from Sigma-Aldrich (U.S.A). Tert-butyl alcohol (TBA), methanol (MeOH), ethanol, and sodium hydroxide (NaOH) were of guaranteed reagent grade and supplied by Daejung Chemical Reagent Company (Korea).
2.2. Synthesis and characterization of CoFe2O4 CoFe2O4 was synthesized using a coprecipitation method modified from the literature24. FeCl3 (0.5 M) and CoCl2 (0.5 M) were mixed in distilled water (DW) with vigorous stirring at 60 °C. After the metal salts had clearly dissolved, NaOH solution in DW (1 M) was added until the pH reached 12. The precipitated layer was separated by centrifugation and washed with DW and ethanol, then dried in a vacuum oven for 12 h at 60 °C. The dried powder was calcined for 12 h at 300 °C in a furnace. The surface morphology of CoFe2O4 was observed using a JSM 7401F (JEOL) high-resolution scanning electronic microscope (HR-SEM). A transmission electron microscope (TEM) image and EDS mapping were obtained using a JEM-2100F (JEOL). The X-ray diffraction (XRD) results were recorded on a Rigaku D/Max-2500 using Cu Kα radiation. The magnetic hysteresis loops of CoFe2O4 were acquired using an MPMS XL-7 (Quantum Design) magnetic property measurement system (MPMS). The X-ray photoelectron spectroscopy (XPS) analysis was performed using an ESCALAB 250 (VG Scientific). The surface properties were obtained using a
Fourier infrared spectroscope (FT-IR, iS50R of Thermo Scientific).
2.3. Batch experiment Batch tests were performed in 80-mL glass vials capped with Teflon septa. A 5mg sample of catalyst was dropped into 50 mL of SMX solution in a standard experiment. An aqueous SMX standard solution of 1.58 mM was prepared, then diluted to 79 M. Then 1 mM of PMS and 0.1 mM of HA were added to the solution. Then 1 mM of PMS and 0.1 mM of HA were added to the solution with initial pH control using HCl and NaOH. The vials were placed on a rolling mixer during the test. At certain time intervals, 1 mL of the samples were withdrawn with a syringe and filtered through a 0.45- m filter. For the reusability test, the used catalyst was separated magnetically after 120 min. Then CoFe2O4 was dispersed in the solution under the same experimental conditions. All treatments were replicated three times.
2.4. Analytical Methods SMX analysis was performed using an Agilent LC/MS system (Agilent 1260 infinity with 6120 quadrupole), which consists of a Luna C-18(2) column, (150ⅹ4.6 mm, 5 m, Phenomenex), and a UV detector coupled with a mass spectrometer detector (MSD). The mobile phase composition was 60:40 (v/v) distilled water and methanol, and the flow rate was 1.0 mL min-1. Under the given conditions, the SMX was eluted at 5.8 min with a maximum absorbance at 265 nm to determine the concentration. In MS detection, the samples were analyzed in APCI-positive mode
electrospray ionization from 50 m/z to 1000 m/z. The analysis method for the concentrations of residual PMS and HA followed previous studies.41, 42 Electron spin resonance (ESR) experiments were performed to measure the reactive radicals with DMPO as a spin-trapping agent using ESR spectroscopy (JESX310, JEOL). The concentration of total organic carbon was obtained using a TOC-V (Shimadzu). The NH4+ , NO3― , and NO2― concentrations were determined using an ion chromatograph (DX-120, Dionex). Total metal concentrations were measured using inductively coupled plasma optical emission spectrometry (ICP-OES, iCAP 7400, Thermo).
3. Results and Discussion 3.1. Characterization of CoFe2O4 The morphology and particle size of synthesized CoFe2O4 were characterized using HR-TEM (Fig. 1a). Most of the particles were aggregated at micrometer scale; the calculated average size of each particle was 36 nm. The prepared particles had polycrystalline structures in the HR-TEM image. The EDX spectrum (Fig. 1a and 1b) indicated that the atomic Co:Fe ratio in CoFe2O4 was 25.0:48.6 which was close to the stoichiometric ratio of 1:2. The XRD pattern (Fig. 1c) of CoFe2O4 particles showed peaks at 2θ = 30.4°, 35.8°, 43.6°, 54.0°, 57.4° and 62.9°, which is corresponded to the Bragg planes (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0), respectively. The Scherrer equation τ=
Κλ 𝛽cos 𝜃
(1)
was applied to the main diffraction peak at 2θ = 35.8° to calculate that the average
crystallite size was 35 nm which was close to the morphological observations. The magnetization characteristics were confirmed using MPMS analysis (Fig. 1d); CoFe2O4 had saturation magnetization MS = 21.6 emu/g. The CoFe2O4 suspension in DW was dark brown, but in the presence of magnetization, it became clear (Fig. 1d, inset images) It means that the synthesized CoFe2O4 was able to separate from the water. The N2 adsorption-desorption by CoFe2O4 showed hysteresis (Fig. S1). The calculated BET area of CoFe2O4 was 23.08 m2/g. These results indicate that nanosized spinel CoFe2O4 ferrite crystals were synthesized, and that they had magnetic properties.
3.2. Catalytic efficiency of the CoFe2O4/PMS/HA system Addition of HA to the CoFe2O4/PMS system increased its SMX-removal effectively (Fig. 2a). In the CoFe2O4 system without HA, approximately 40% of SMX was decomposed within 120 min (k = 5.5ⅹ10-3 min-1); the slow degradation rate might be due to the low PMS activation efficiency of the Fe3+ in the CoFe2O4 structure. On the other hand, the addition of HA greatly increased the reactivity, and the CoFe2O4/PMS/HA system degraded most of the SMX within 120 min (k = 3.4ⅹ10-2 min-1). These results indicate that both CoFe2O4 and HA promote the activation of PMS removal of SMX in the CoFe2O4/PMS/HA system. When only CoFe2O4 was present, 12% of SMX was eliminated by adsorption to the surface of CoFe2O4. In addition, HA in the absence of PMS did not affect the degradation of SMX, but HA itself could activate PMS slightly. During the reaction, the residual PMS in the system was reduced to 40% of the initial concentration, but 15% of
reduced concentration is due to natural decrease, such as adsorption and dissolution of PMS, which are not related to the contaminants degradation (Fig. S12). Plus, residual HA concentration was measured indirectly by the addition of acetone. After the reaction, about 40% was decomposed, showing no selectivity for the target contaminant SMX (Fig. S13). The composition of cobalt ferrite was investigated to affect the SMX removal with PMS and HA (Fig. 2b). The CoFe2O4 with HA showed a faster reaction rate than that of Co0.5Fe2.5O4 (k = 1.7ⅹ10-2 min-1). However, the rate of SMX degradation by Fe3O4 (k = 2.2ⅹ10-2 min-1) was still slower than by CoFe2O4. This result shows that Co2+ in spinel ferrite increases its ability to activate PMS. Furthermore, Fe3O4 has a reaction rate that is similar to the reaction rate of Fe2O3 which has only Fe3+ as an iron source (k = 2.2ⅹ10-2 min-1). Based on the results, the increase in SMX removal by HA is more pronounced with Fe3+ than with Co2+. This difference was obvious when compared with experiments that did not include HA (Fig. S2). To select target compounds for this study, we compared the rates at which various organic pollutants were degraded. Within 120 min, the CoFe2O4/PMS/HA system degraded 46% of BPA, 65% of CBZ, 100% of SMX and 100% of DCF (Fig. S3a). DCF was well removed even without the addition of HA, so SMX was used as a model compound (Fig. S3b). The catalytic activity the CoFe2O4/PMS/HA system increased as the calcination temperature decreased; calcination of CoFe2O4 at 150 °C was the ideal experimental condition for PMS activation (Fig S4). However, calcination at higher
temperatures reduced the cobalt elution26, so the experiments were conducted using the CoFe2O4 particles that had been calcined at 300 °C.
3.3. Effects of operating parameters for the CoFe2O4/PMS/HA system Various experimental conditions affected the catalytic activity of the CoFe2O4/PMS/HA system (Fig. 3). With the constant initial PMS concentration (79 μM), the degradation of SMX by the CoFe2O4/PMS/HA system was dependent on the CoFe2O4 concentration (Fig. 3a). SMX was completely degraded at fixed [CoFe2O4] = 100 mg/L, but only 75.3% was degraded at the SMX [CoFe2O4] = 10 mg/L. As [CoFe2O4] increased, the active sites on the surface also increased, so the PMS activation and SMX removal rates increased. The PMS concentration also affected the SMX degradation in the CoFe2O4/PMS/HA process (Fig. 3b). Increase in the PMS concentration from 0.2 to 2.0 mM caused increase in the SMX degradation kinetics. The linear correlation between the PMS concentration and the reaction rate would be a result of the number of radicals generated by PMS. The efficiency of SMX degradation increased as the HA concentration increased from 0.1 to 1.0 mM, but the rate of increase in degradation rate diminished at [HA] >2.0 mM. (Fig. 3c). These results indicated that HA could exist in the solution as a reducing agent at concentrations < 1.0 mM, but not at higher concentrations, due to the competition with SMX.25
pH, also was shown to affect the SMX degradation in the CoFe2O4/PMS/HA process (Fig. 3d). When the initial pH of the solution was approximately 3, the SMX removal was significant in the CoFe2O4/PMS/HA process due to the dissolution of metal cations into the solution.26. As the pH increased from 3 to 9, the reaction rates decreased from 3.5ⅹ10-2 min-1 to 2.3ⅹ10-2 min-1 because the pH affected the speciation of HA (NH3OH+, NH2OH, and NH2O-), which influenced the efficiency of the electron transfer27. Furthermore, as the pH of the solution increased, the selfdecomposition of PMS increased, so the efficiency of SMX degradation decreased.28
3.4. Metal leaching effects of the CoFe2O4/PMS/HA system To
evaluate
the
homogeneity/heterogeneity
of
the
reaction
in
the
CoFe2O4/PMS/HA system, the concentration of leached metal ions was determined after reactions (Fig. 4a). It was shown that Co was leached more than Fe, although the CoFe2O4 structure has two times of Fe than Co. After 120 min, the concentration of leached Fe was 0.15 mg L−1 which was only 0.09–0.21% of the total Fe in the system, whereas the concentration of leached Co was 0.20 mg L-1 which was not much different from a previous toxicity study of CoFe2O4.29, 30 Based on the leached metal conditions, we compared the contribution of radical reactions occurring on the surface and the reaction in the solution. The eluate, in which CoFe2O4 was leached in D.I. water for 2 hours, was used with HA and PMS. After the reaction in a homogeneous system, around 75% of SMX was degraded compared to the adsorption and heterogeneous degradation (Fig. 4b). This result
indicated that the effects of the leached iron and cobalt were greater than the reaction on the surface.
3.5. Surface transformation of the CoFe2O4/PMS/HA system Elemental composition of the surface of CoFe2O4 with HA was investigated using XPS. The full scan survey of the samples presented Fe, Co, O, and C peaks (Fig. S5). Detailed XPS data (Fig. 5a) of the Fe 2p level peak showed a main peak at ~ 710.2 eV which corresponded to CoFe2O4, and binding energies of 711.5 eV and 709.0 eV that were assigned to Fe3+ and Fe2+, respectively.31,
32
The reaction without HA caused
surface oxidation, so the binding energy increased compared to the material before the reaction. In the reaction with HA, the binding energy did not change, suggesting that surface reductions might be occurred. The Co 2p XPS spectrum (Fig. 5b) suggests that the spectrum is relevant to the CoFe2O4 samples and indicates similar trends with the Fe 2p level after the reaction. The areas of the Co 2p and Fe 2p peaks were calculated and used to estimate the ratios of the cation concentration of CoFe2O4 (Table S1). The results demonstrated that Fe3+ and Co2+ were definitely transformed to Fe2+ and Co3+ with HA, whereas the ratios of Fe3+ and Co2+ increased in the system without HA. This result indicates that HA promoted the electron transfer in the Fe3+/Fe2+ and Co2+/Co3+ cycles. FT-IR was used to further assess the surface of CoFe2O4 after reaction in the CoFe2O4/PMS/HA system (Fig. S6b). In the scope of 580 cm−1 wavenumber, the CoFe2O4 displayed distinct peaks of Fe-O. In addition, the CoFe2O4 particles after the
reaction with HA exhibited a relatively weak Fe-O peak. After the reaction, the 3405 cm-1 band, which coincides with hydroxyl groups, appeared because of the presence of water. In addition, the clarity of the baseline confirmed that the HA and degradation products did not adsorb on the surface of CoFe2O4 during the reaction. The EDX results (Fig. S7) of CoFe2O4 after the reaction in the CoFe2O4/PMS/HA system showed that the atomic ratio of Co:Fe in the particle was 1:0.33, which meant that more Co than Fe was leached, as interpreted from Fig. 4. The FT-IR spectra and XPS results confirmed that the nitrogen from HA did not adsorb on the surface of CoFe2O4.
3.6. Identification of reactive species Two radical scavengers of methanol (MeOH) and tert-butanol (TBA) were used to distinguish the active radical species in the CoFe2O4/PMS/HA system. The scavengers were mixed with the solution at a sufficient concentration (1 M) to ensure preference between the radicals and the scavengers. The reaction rate constants between the radicals with MeOH (kSO4•− = 3.2 ⅹ106 M-1 s-1; k∙OH = 9.7 ⅹ 108 M-1 s-1) and TBA (kSO4•− = 4.0 ⅹ105 M-1 s-1; k∙OH = 6 ⅹ 108 M-1 s-1) were different33, so the functions of SO 4― • and •OH could be distinguished. The SMX-removal efficiency was reduced from 99.0% to 56.7% when MeOH was added, and to 75.7% when TBA was added (Fig. 6a). This result indicated that both SO 4― • and •OH occurred in the CoFe2O4/PMS/HA system but that SO 4― • contributed more to the degradation of SMX than •OH. In the system without HA, the degradation efficiency of SMX
decreased from 45.4% to 31.7% when MeOH was added, and to 42.3% when TBA was added (Fig. S8). This result means that •OH seems to have no significant effect on the PMS activation if HA is not present. In other words, the amount of •OH increases when HA is added in the CoFe2O4/PMS system. Hence, the addition of HA aids electron transfer of the radical pathways in the system and indirectly promotes the generation of SO4 and OH radicals To test this hypothesis, the active radical species in the system were detected using EPR. The radicals were captured using DMPO which is a spin-trapping agent. Overlapping DMPO-SO4 and DMPO-OH signals appeared in the CoFe2O4/PMS/HA system (Fig. 6b). These signals indicated that both SO 4― • and •OH existed in the PMS aqueous solution, confirmed by Fig. 6a. However, without HA, the DMPO-SO4 and DMPO-OH signals were not detected until 5 min after the addition of CoFe2O4. These DMPO-SO4 and DMPO-OH could be attributed to the multiplied •OH radicals as: 4,20,34
HSO5- + H2O ↔ H2O2 + HSO4-
(2)
H2O2 + Fe2+ → Fe3+ + •OH + OH-
(3)
When HA was added to the solution, the DMPOX signal was disappeared; this change indicates that the excessive •OH radicals that oxidized DMPO to DMPO-SO4 and DMPO-OH were easily removed by the regenerated Co2+: Co2+ + HSO5- → Co3+ + OH- + SO4-• .
(4)
The results of the radical-scavenger experiments demonstrate that the • OH radicals did not affect the degradation efficiency in the system when HA was not
present. Therefore, the excessive •OH radicals generated in the system without HA could not affect the pollutant degradation.
3.7. Determination of degradation intermediates According to the TOC removal data (Fig. S10b), the mineralization efficiency of SMX was lower than the total SMX removal amount, indicating that the mineralization was incomplete as a result of the formation of intermediate products. To propose the degradation pathway (Fig. 7), LC-MS experiments were conducted (Fig. S9) showing eight intermediate peaks [M + H]+ at m/z = 254, 185, 173, 143, 100, 99, 60 and 59. The structures of these intermediates were suggested by examining the MS fragmentation patterns, in which the degradation pathway from SMX to small organic compounds was hypothesized (Fig. 7). First, the S-N bond of SMX was cleaved, followed by dissociation of the other bonds and modification of the functional groups of the benzene and isoxazole rings, depending on the reactive radicals present. IC was also used to quantify the NH 4+ , NO 3― , and NO 2― that the reaction generated (Fig. S10a). NH 4+ and NO 3― , were produced rapidly within 60 min from the degradation products of SMX and HA. The concentration of NO 3― did not change after 60 min, but the concentration of NH 4+ gradually decreased as a result of adsorption. The amount of NO 2― was insignificant during the degradation process compared to the NH 4+ and NO 3― , because NO 2― is easily oxidized to NO 3― .
3.8. Reaction mechanism of the CoFe2O4/PMS/HA system From the surface, water molecules are adsorbed to the ≡Co and ≡Fe sites to produce ≡ Co2+ − −OH from ≡ Co(OH)+ and ≡ Fe3+ − −OH
36,37.
When PMS was
dropped into the solution, the ≡Co2+ and ≡Fe3+ of the surface reacted with PMS to generate •OH (Eqs. 5, 6). Then the ≡Co2+ − −OH on the CoFe2O4 surface activated PMS to generate bound SO 4― • and •OH (Eq. 7), and more ≡ Co2+ − −OH were generated (Eq. 8). Similarly, the ≡Fe2+ − −OH and ≡Fe3+ − −OH could also generate the surface-bound SO 4― • radicals from HSO5- (Eqs. 9, 10). The reported standard reduction potential of Co 3+ /Co 2+ redox pair is 1.81 V and that for Fe 3+ /Fe 2+ is 1.51 V, whereas the reduction reaction of HA is -1.87 V (Eq.11) 35. This comparison reveals that the regeneration of Co2+ and Fe2+ on the surface is thermodynamically favorable (Eq. 12). Furthermore, the regenerated ≡Co2+ decreased the excessive •OH radicals that might contribute to the self-decomposition of PMS, as ESR results indicated (Eq. 4). The SMX in the aqueous solution unceasingly reacted on the surface of CoFe2O4 and was degraded by SO4•− and •OH (Eq. 13). ≡ Co2+ + HSO5−→ ≡Co3+ + SO42−+ •OH
(5)
≡ Fe2+ + HSO5−→ ≡Fe3+ + SO42−+ •OH
(6)
≡ Co2+ − −OH + HSO5−→ ≡Co3+ − −OH + SO 4― • + OH−
(7)
≡ Co3+ − −OH + HSO5−→ ≡Co2+ − −OH + SO 4― • + H+
(8)
≡ Fe2+ − −OH + HSO5−→ ≡Fe3+ − −OH + SO 4― •
(9)
≡ Fe3+ − −OH + HSO5−→ ≡Fe2+ − −OH + SO 4― •+ H+
(10)
≡Fe3+/≡ Co3+ ≡Fe2+/≡ Co2+
(11)
2NH3OH+ → N2 + 2H2O + 4H+ + 2e-
(12)
SO 4― • (or •OH) + Organic pollutants →...→ CO2 + H2O
(13)
From the above reactions, we could hypothesize overall reaction mechanism of the SMX degradation in the CoFe2O4/PMS/HA system(Fig. 8).
3.9. Feasibility and reusability of the CoFe2O4/PMS/HA system Three water samples (Table S2) were used to test the integrated effects of the background on the degradation of SMX (Fig. 9a). For wastewater sample, the efficiency of SMX degradation was 87.5%, which was lower than the experiment in the distilled water because the chloride anions inhibited the PMS activation.38 In the groundwater sample #1, the efficiency of SMX degradation was similar to that with the distilled water in the HA containing system and only 42% without HA. However, in groundwater sample #2 the efficiency of SMX degradation was 98% with HA, which was not much different from the degradation efficiency of the system without HA (Fig. S11); this similarity might be the result of the high redox condition of the groundwater sample #2 (Eh = -132.3). In order to mimic the real environment, an additional experiment with a reduced initial SMX concentration (4 μM) was conducted. The degradation rate was slower than the control, however SMX was perfectly removed after 120 min. Six cycles of the same CoFe2O4 material were performed to estimate the reusability with the addition of HA. The efficiencies of SMX removal after 120 min
remained as high as 86.5% after six cycle runs, whereas the removal efficiency with Fe0 decreased to 53.5% (Fig. 9b). This result indicated that HA could help to maintain the reactivity CoFe2O4 against oxidation of its active catalytic sites (≡Co3+ and ≡Fe3+).
4. Conclusion
A novel CoFe2O4/PMS/HA system to effectively activate PMS for SMX degradation was devised. This system, which used environmentally benign reductant, degrades SMX much better than the conventional CoFe2O4/PMS system. The presented system was optimized for different pH values and concentrations of PMS, HA and NPs. Compared with conventional AOPs, which shows the optimum condition only in acidic pH, the CoFe2O4/PMS/HA system showed broad pH range to be applied even in neutral pH condition. Addition of HA to the CoFe2O4/PMS system significantly promoted the Fe3+/Fe2+ and Co2+/Co3+ cycles on the surface of CoFe2O4 by thermodynamically-favorable reaction with HA. Especially, HA reduced the selfdecomposition of the •OH radicals generated by Co2+/PMS. In other words, this system could maximize the efficiency of sulfate radical and minimize the disadvantage of cobalt based PMS activation. Furthermore, LC/MS analysis and IC were measured to provide detail degradation mechanisms of SMX. The LC/MS analysis suggested that most of the intermediates were generated by cleavage of S-N bonds, and IC results indicated trace amount of ionic byproducts. The proposed system showed good feasibility in actual water samples and maintained high
reactivity during reusability tests, which means the system could be applied in real remediation system.
Acknowledgement This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. 2017R1A2B3012681).
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a)
b)
Fe
Co Fe Fe Co O
Co
0
c)
d)
(311)
(220) (511)
(400)
Magnetization (emu/g)
Intensity
CoFe2O4
(440)
(422)
5
10
15
20
20 CoFe2O4 10
0
-10
-20 20
30
40
50
2 theta (degree)
60
70
-10
-5
0
5
10
Magnetic Field (kOe)
Fig. 1. Characterization of the fabricated spinel CoFe2O4. a), b) HR-TEM images with EDX spectrum, c) XRD pattern, and d) magnetic hysteresis loop. Embedded image: CoFe2O4 suspension under no magnetization (left) and under magnetization (right).
a) 1.0
C/C 0
0.8
0.6 CoFe2O4 PMS+CoFe2O4
0.4
PMS+HA+CoFe2O4 Control HA+PMS HA+CoFe2O4
0.2
0.0 0
20
40
60
80
100
120
Time (min)
b) 1.0 Co0.5Fe2.5O4 CoFe2O4
0.8
Fe3O4
C/C 0
Fe2O3 Co3O4
0.6
0.4
0.2
0.0 0
20
40
60
80
100
120
Time (min)
Fig. 2. a) Degradation efficiency of sulfamethoxazole in CoFe2O4/PMS/HA system, and b) SMX degradation with various Co-Fe mixed oxides. Experimental conditions: [Pollutants]0 = 79.0 μM, [Oxides] = 0.1 g/L, [PMS]0 = 1 mM, [HA]0 = 0.1 mM, and pH = 5.
a)
b) 1.0
1.0
0.8 0 mg/L 10 mg/L 50 mg/L 100 mg/L 200 mg/L
0.6
0.4
C/C 0
C/C 0
0.8
0.2 mM 0.5 mM 1 mM 2 mM
0.6
0.4
0.2
0.2
0.0
0.0 0
20
40
60
80
100
120
0
20
40
Time (min)
80
100
120
d)
c) 0 mM 0.02 mM 0.05 mM 0.1 mM 0.2 mM 0.5 mM
1.0
0.8
0.6
1.0
0.6
0.4
0.4
0.2
0.2
0.0
pH=3 pH=5 pH=7 pH=9
0.8
C/C 0
C/C 0
60
Time (min)
0.0 0
20
40
60
Time (min)
80
100
120
0
20
40
60
80
100
120
Time (min)
Fig. 3. Effects of a) particle dosage, b) PMS concentration, c) hydroxylamine concentration, and d) pH on the degradation of SMX in CoFe2O4/PMS/HA system. Experimental conditions: [SMX]0 = 79.0 μM, [CoFe2O4] = 0.1 g/L, [PMS]0 = 1 mM, [HA]0 = 0.1 mM, and pH = 5.
a)
b) 1.0
C/C0
0.8 Adsorption Homogeneous PMS+HA+CoFe2O4
0.6
0.4
0.2
0.0 0
20
40
60
80
100
120
Time (min)
Fig. 4. a) Total leached metal concentrations in 120 min., b) Degradation of sulfamethoxazole in homogenous condition after CoFe2O4 is leached out.
Experimental conditions: [SMX]0 = 79.0
μM, [CoFe2O4] = 0.1 g/L, [PMS]0 = 1 mM, [HA]0 = 0.1 mM, and pH = 5.
a)
Fe 2p
Before reaction
Fe3+
Fe2+
Intensity
Without HA
With HA
735
730
725
720
715
710
705
Binding Energy (eV)
Co 2p
b)
Co2+
Intensity
Before reaction
Co3+
Without HA
With HA
810
805
800
795
790
785
780
775
Binding Energy (eV)
Fig. 5. XPS spectra of the a) Fe 2p and b) Co 2p regions for fresh CoFe2O4 samples and treated CoFe2O4 samples with or without hydroxylamine.
a) MeOH 1 M MeOH 2 M TBA 1 M TBA 2 M BQ 1 M Control
1.0
C/C 0
0.8
0.6
0.4
0.2
0.0 0
20
40
60
80
100
120
Time (min)
b)
Control
Intensity
Without HA
With HA
332
334
336
338
Magnetic field (mT)
Fig. 6. a) Effect of radical scavengers on SMX degradation, and b) ESR spectrum in a CoFe2O4/PMS/HA system with a DMPO (0.05 M) spin-trap. Experimental conditions: [SMX]0 = 79.0 μM, [CoFe2O4] = 0.1 g/L, [PMS]0 = 1 mM, [HA]0 = 0.1 mM, and pH = 5.
Fig. 7. Proposed degradation pathway of SMX in a CoFe2O4/PMS/HA system.
Fig. 8. Proposed reaction mechanism for the SMX degradation by a CoFe2O4/PMS/HA system.
a) 1.0
Wastewater Groundwater #1 Groundwater #2 Groundwater #2 (4 m SMX) Distilled water
C/C 0
0.8
0.6
0.4
0.2
0.0 0
20
40
60
80
100
120
Time (min)
b)
1.0 0.8
C/C 0
0.6 0.4 0.2 Fe0+PMS+HA CoFe2O4+PMS+HA
0.0
0
200
400
600
Time (min)
Fig. 9. a) Feasibility test in actual water body with spiked SMX and b) reusability test for CoFe2O4/PMS/HA system during six cycles. Experimental conditions: [SMX]0 = 79.0 μM, [CoFe2O4] = 0.1 g/L, [PMS]0 = 1 mM, [HA]0 = 0.1 mM, and pH = 5.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
There are no potential competing interests.
Highlights Hydroxylamine(HA) accelerated peroxymonosulfate(PMS) activation with CoFe2O4. Co(II)/Fe(III) cycle on the surface of CoFe2O4 was promoted by HA. Sulfate radical was major reactive substance in CoFe2O4/PMS/HA syste m. Pathway of sulfamethoxazole degradation was investigated using LC-MS and IC.