Insight into reactive oxygen species in persulfate activation with copper oxide: Activated persulfate and trace radicals

Accepted Manuscript Insight into reactive oxygen species in persulfate activation with copper oxide: activated persulfate and trace radicals Xiaodong Du, Yongqing Zhang, Imtyaz Hussain, Shaobin Huang, Weilin Huang PII: DOI: Reference:

S1385-8947(16)31547-9 http://dx.doi.org/10.1016/j.cej.2016.10.138 CEJ 15994

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

7 August 2016 4 October 2016 31 October 2016

Please cite this article as: X. Du, Y. Zhang, I. Hussain, S. Huang, W. Huang, Insight into reactive oxygen species in persulfate activation with copper oxide: activated persulfate and trace radicals, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.10.138

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Insight into reactive oxygen species in persulfate activation with copper oxide: activated persulfate and trace radicals Xiaodong Du a, Yongqing Zhanga,b,c,d*, Imtyaz Hussain a, Shaobin Huanga, Weilin Huange a

School of Environment and Energy, Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, South China University of Technology, Guangzhou 510006, China.

b

The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, Guangzhou 510006, China.

c

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China.

d

State Key Laboratory of Subtropical Building Sciences, South China University of Technology, Guangzhou 510641, PR China

e

Department of Environmental Sciences, Rutgers, The State University of New Jersey, New Brunswick, NJ 08901, USA

* Corresponding author: Yongqing Zhang

Tel: +86-20-39380569

Fax: +86-20-39380508

E-mail address: [email protected]

ABSTRACT

Activated persulfate along with trace sulfate and hydroxyl radicals were found to be reactive oxygen species in copper oxide activated persulfate system in this study. Persulfate decomposition study revealed that activated persulfate was predominant instead of sulfate and hydroxyl radicals. The radical scavenger investigation supported this point and led to the surface activation mechanism of persulfate. The p-chloroaniline (PCA) degradation in copper oxide activated persulfate system in neutral and acidic condition provided the evidence of prime heterogeneous persulfate activation and minor homogeneous persulfate activation caused by dissolved copper ion. Electron Paramagnetic Resonance study indicated the generation of sulfate and hydroxyl radicals. Integrating all the experiment facts, it is proposed that activated persulfate was main reactive oxygen species generated on copper oxide surface, along with minor sulfate and hydroxyl radicals produced. Chloride ions showed no effect on PCA degradation in copper oxide activated persulfate system. While bicarbonate ions improved PCA degradation without pH adjustment (initial pH is about 3) and performed an inhibition effect when the initial pH was adjusted to neutral. These findings indicated the potential applicability of copper oxide for in situ chemical oxidation for two reasons: (1)ubiquitous chloride and bicarbonate ions showed no negative effect; (2) the slight radical generation avoid the persulfate consumption by non-pollutant reductive materials.

Keywords: EPR

persulfate, partial activation, p-chloroanilne, hydroxyl radical, sulfate radical,

1. Introduction Persulfate-based advanced oxidation is a promising technique for destructive removal of organic pollutants from water and soils[1-4].

The process involves generation of sulfate and

hydroxyl radicals that are highly reactive and nonspecific selectivity for oxidation of organic molecules[1, 5, 6]. Heat[7, 8], UV and visible light[9-11], alkali[12, 13], transition metal ions[14-16] and heterogeneous activators[17-20] are efficient for activating persulfate to produce sulfate and hydroxyl radicals. Persulfate tends to obtain an electron to form a sulfate radical which may further react with water and hydroxyl ions to produce hydroxyl radicals[12]. In the persulfate oxidation systems, sulfate radicals are the main species in acid and neutral conditions, but hydroxyl radicals may dominate in basic solutions when persulfate is activated by base[21] or heterogeneous activators[22]. Heterogeneous activators are ideal activators for persulfate activation which show considerable efficiency, less chemicals or energy requirement and continuous activation performance. Zero valent iron works well in persulfate activation by continuously providing

ferrous ions which activate persulfate to form sulfate radicals[19, 23-26]. Minerals such as pyrite, clay and other tested species successfully activate persulfate to degrade organic pollutants[27-29], which demonstrates the feasibility of in situ chemical oxidation (ISCO). Transition metal oxides have received great attention since both the forms of oxides and relevant transition metal ions activate persulfate efficiently. Researchers have tried iron oxides[30-32], zinc oxides[33], manganese oxides[34] and titanium dioxide[35, 36] to activate persulfate and received effective pollutants degradation. In the field of metal-absent materials, carbon with kinds of morphological features like nanotubes[37], granular[38], diamond crystal[39], biochar[40] and so on were also applied to activate persulfate. Without exception, these catalysts activate persulfate by generating sulfate radicals. Recently, copper oxide has been paid attention into as a suitable heterogeneous activator for activating persulfate because of its little energy input, lower cost of pH adjustment and evitable heavy metals contamination and high treatment efficiency at neutral condition[41, 42]. There are two different mechanisms proposed recently[42, 43]. Zhang[42] reported the non-radical surface activation mechanism of persulfate with copper oxide. They found sulfate and hydroxyl radical scavengers—ethanol had no negative effect on 2,4-DCP degradation in copper oxide activated persulfate system, which opposite to radical oxidation process. Furthermore, the Attenuated Total Reflection Fourier Transform Infrared spectrum (ATR-FTIR) and Confocal Raman applications on mixture of persulfate and copper oxide implied no persulfate decomposition and no chemical bond formed on surface, which supported the non-radical generation process. However, Lei[43] reported that the persulfate activation by copper oxides followed a radical mechanism and the main oxidative species

were hydroxyl and sulfate radicals absorbed on CuO-Fe3O4 surface in CuO-Fe3O4 activated persulfate system. They proposed that surface Cu(II) went through a Fenton-like reaction with persulfate to form Cu(III) and sulfate radicals, both of which could subsequently reacted with water to produce hydroxyl radicals. It should be noticed that iron species such as zero valent iron (ZVI), pyrite, iron oxide and ferrous ion are efficient persulfate activators to promote radical generation. So in Lei’s system[43]，Fe3O4 was also responsible for activating persulfate. The activation mechanism in CuO-Fe3O4/PS system might be effected by iron species. Thus, apparently, the mechanism of copper oxide activated PS process is still disputable. The objectives of this work were to further explore the mechanism and applicability of copper oxide activated persulfate system for refractory pollutants degradation. PCA (p-chloroaniline) was chosen as target pollutant. PCA is suspicious carcinogenic, high toxic and persistent. In addition, it is one of the wide used intermediates and raw materials of dyes, pesticides, plastics, cosmetics and pharmaceutical production, which can be released to environment from producing, processing and utilizing, forming pollution and resulting in the risk of ecology and human health welfare losses[44]. More physiochemical properties could be refer to Text S1. To rule out the influence of iron, pure copper oxide was selected as catalysis. The main reaction oxidation species, PCA degradation efficiency and copper oxide recycle use stability were investigated.

2. Materials and methods

2.1. Chemicals. Methanol (HPLC, 99.9%) and p-chloroaniline (AR, 98%) were purchased from Sigma-Aldrich Chemical Company (Shanghai, China). Phenol, ethanol, 5, 5-Dimethyl-1-pyrroline N-oxide (DMPO, AR, ≥96.0%) were purchased from Tianjin Nuoke Science and Technology Development Ltd (Tianjin, China), Shanghai Titan Company (Shanghai, China) and Shanghai Aladdin Company (Shanghai, China), respectively. Na2S2O8, KI, Cu(NO3)2·3H2O and NaCl at their highest grades were purchased from Tianjin Damao Chemical Reagent Factory (Tianjin, China). NaOH was purchased from Tianjin Baishi Chemical Ltd (Tianjin, China).

Na2S2O3, NaHCO3 and H2SO4 were purchased from

Guangzhou Chemical Reagent Factory (Guangzhou, China). Water used in the experiment was deionized distilled water produced by a Millipore Milli-Q system (America). Copper oxide preparation. Copper oxide was obtained following a thermal treatment procedure described in Liang’s paper[41].

In brief, 5g of Cu(NO3)2·3H2O in a ceramic o

crucible was placed in a muffle furnace. The temperature was elevated to 150 C in 10 min, and retained there for 1 hour. hours.

o

It was increased to 300 C in 10 min, and retained there for 4

The final product was collected after the furnace was cooled to room temperature

and stored in a desiccator before use. 2.2.Experimental procedure. The oxidation reaction was conducted at 20 ℃ using a completely mixed batch reactor -1

system. An initial aqueous solution was prepared by diluting 5 mL 10 mmol·L PCA solution with 90 mL deionized water. The reactor was closed and shaken at 125 rpm for 10 min. An aliquot of 0.5 mL mixture was sampled and filtered through 0.45 µm m filters. filters Then the mixture

-1

was further mixed with 0.5mL sodium thiosulfate solution (2 mol·L , to keep the same sampling and test condition with calibration curve and samples with persulfate) for analyzing the initial PCA concentration. Then 0.05 g copper oxide was added, the reaction was initiated -1

by adding 5 mL 50 mmol·L sodium persulfate to the reactor. The content of the reactor was continuously mixed on a shaker.

An aliquot of 0.5 mL reaction solution was sampled and

filtered through 0.45 µm m filters at specific time interval, mixed with 0.5 mL sodium thiosulfate solution (2 mol·L-1) for quenching the reaction, and analyzed for residual PCA concentrations. To void the volume dilution effect, the calibration curve was made at the same condition. To quantify the effect of bicarbonate and chloride ions on the rates, a certain amount of bicarbonate, sodium chloride were respectively added into CuO/PS/PCA system -1

(bicarbonate and chloride ions concentration were both three gradient: 2.5, 5 and 10 mmol·L ). To identify the radical species, ethanol and phenol were respectively added into CuO/PS/PCA system (the ethanol and phenol concentration was both 500 mmol·L-1). 1 mL -1

0.1 mol·L DMPO was added into 10 mL reaction mixture (the mixture had been reacting for 5min before the addition) to react for 1 min before EPR analysis. The initial pH of reaction mixtures were adjusted by sulfuric acid and sodium hydroxide solution. 2.3.Analysis methods. Copper oxide prepared and used in this study was characterized with an SEM (Scanning Electron Microscope, equipped with Energy Dispersive Spectrometer (EDS)) EVO 18, XRD (X-ray diffraction) Bruker advance 8 and XPS (X-ray photoelectron spectroscopy) Phi X-tool instrument. The aqueous concentrations of PCA were analyzed on a Shimadzu LC-20A HPLC(High Performance Liquid Chromatography) equipped with a diode array detector set

at 254 nm wavelength to monitor the signal and with a reverse-phase C18 column. The HPLC was run using a water and methanol mixture at a volumetric ratio of 30:70 as the mobile phase. The flow rate was set at 0.7 mL/min and the oven temperature was at 40 oC. The degradation curves of PCA have been fitted and follow pseudo-first order, the results are showed in Table. S1.

Persulfate concentration was determined by spectrophotometry

developed by Liang. Aqueous concentrations of copper were analyzed on Shimadzu AA-6880 AAS (Atomic Absorption Spectrophotometer). The sulfate and hydroxyl radicals were identified on an EPR (Electron Paramagnetic Resonance) Spectrometer Bruker A300, which was set up as follows: microwave frequency at 9.8752 GHz, sweep width at 100 G, center field at 3504.12 G, modulation amplitude at 1 G, time constant at 327.68 ms, and scan time of 40 s.

3. Results and discussion 3.1.Characterization of copper oxide Copper oxide was characterized by chemical analysis, EDS, SEM , XRD and XPS. Chemical analysis (Atomic Absorption Spectroscopy determination of the copper ion concentration by dissolving certain amount of copper oxide in deionized water) showed that the copper oxide contained 77.7 wt.% of Cu, compared favorably to the stoichiometric value of 80% and Energy Dispersive Spectrometer (EDS) result of 73.1% (Table. S2). The EDS result revealed that copper density on surface was lower than that in bulk and more oxygen atoms were present on surface. Fig. 1 presents SEM images of copper oxide prepared and used in this study. The solids had spherical shape with sizes ranging from 10 µm to about 100

µm. At higher resolutions, the solids appeared to be aggregates of elongated crystals with rough surfaces (SI, slide 1). XRD spectra in Fig. 1 indicated that copper oxide was monoclinic. The spectrum was hit by three PDF cards namely #89-5896, #89-5899 and #80-1917 with 23 peaks matched (SI). The peak fitting result indicated the high crystallinity (97%, SI) of the material, which characterized this heterogeneous activator an order structure. For the characterization of the CuO after reaction, the results are presented in supplementary information. After being used, the copper oxide became cleaved, agminated and more rough (Fig.S1-S3, SI slide 2), the elongated crystals were broken into smaller pieces. XRD results showed that no crystal structure change was observed before and after reaction, indicating that CuO could retain its structure to be stable. XPS results showed that there was no valent change of Cu on CuO surface before and after reaction, which suggested that CuO oxide was a true catalyst compared with irreversible activator like Fe2+, Fe0 and FeS2. The formation of CuO-PS transition state make it more easily to PCA oxidation by PS, the trace unstable Cu(III) formed from Cu(II) oxidation by PS would transformed into Cu(II) fast.

Fig. 1. SEM images and XRD pattern of copper oxide.

3.2.Persulfate activation efficiency by copper oxide and proposed mechanism. Three reaction systems were compared, including PCA with copper oxide (PCA/CuO), PCA with sodium persulfate (PCA/PS) and PCA with both copper oxide and sodium persulfate (PCA/PS/CuO). Results (Fig. 2a) showed that PCA/PS/CuO system achieved the most PCA removal with 71.5%, which agreed with previous work[41]. While only 4.9% of PCA was removed in PCA/PS system and nearly no PCA was removed in PCA/CuO system, which indicated the limited PCA adsorption by copper oxide and slight PCA oxidation by persulfate alone. Therefore, it was concluded that the copper oxide could enhance the oxidation ability of persulfate. For further research on persulfate activation mechanism, persulfate decomposition study was carried out. Result presented that persulfate was hardly

decomposed in PS/CuO system in absence of PCA (Fig. 2b). This process was different from the activation of PS by Fe2+, in which persulfate was well consumed by Fe2+ 55. In the Fe0-PS system, the decomposition efficiency of PS is still high without PCA23. However, with copper oxide present, only after the addition of PCA the persulfate would significantly decompose(Fig. 2b). Considering the 13.2% persulfate consumption in PS/PCA system, we concluded that persulfate was activated by copper oxide (Fig. 2b). The results demonstrated that persulfate hardly dissociated and sulfate radical probably not generate when activated by copper oxide, since the generation of sulfate radicals would induce persulfate consumption by initiating radical propagation reaction. To further verify this conclusion, experiments of ethanol and phenol (both are good sulfate and hydroxyl radical quenchers) decomposition by PS/CuO system have been done, and the results showed that nearly no ethanol and phenol was decomposed (Fig. S4, S5). Therefore it was proposed that copper oxide could activate persulfate without radical generation. The reactive oxygen species was still the persulfate but not radicals. When the electron-rich source PCA was introduced, activated persulfate obtained electrons from PCA and decomposition occurred.

a

b

Fig. 2. The rate of PCA removal in the three reaction systems: CuO/PCA, PS/PCA and CuO/PS/PCA (a), persulfate degradation with and without PCA addition : PS/CuO, PS/PCA, PS/CuO/PCA systems, pH has been adjusted to 7 (b). Conditions: [PS]0 = 2.5 mmol·L-1, [PCA]0 = 0.5 mmol·L-1, [CuO] = 0.5 g·L-1. 3.3.Identification of reactive oxygen species in copper oxide activated persulfate solution 3.3.1. Radical scavenger study. Several scavengers were used to identify sulfate and hydroxyl radicals in solution in previous studies (Table 1). Ethanol and phenol were used as sulfate and hydroxyl radical scavengers in this study. In order to test the quenching effect of ethanol and phenol for persulfate at 20℃, experiments of reaction between ethanol and persulfate, phenol and persulfate have been done, the results indicated that persulfate won’t be consumed by ethanol and phenol at 20℃ (Fig. S6).Results (Fig. 3) showed that phenol had much more significant negative effect on PCA degradation than ethanol. The PCA residual after 300 min reaction was 77.4 and 88.4% at pH 7 and pH 3 (no pH adjustment) respectively with 500 mmol·L-1 -1

phenol addition, while it was 47.8 and 49.6% with 500 mmol·L ethanol addition. Considering 28.5 and 35.9% PCA residual in system without scavengers, it is proposed that radical oxidative species were not the main reactive oxidative species with PCA since ethanol had limited PCA degradation inhibition. While the significant PCA degradation inhibition by phenol implied that the main activation process was occurred on copper oxide surface and phenol molecule obstructed the contact between persulfate ion and oxide surface. Since the dielectric constants of phenol and ethanol were 12.4 ( 303.2K ) and 25.3 ( 293.2K ) ( compared with water 80.1 at 293.2K )[45], phenol molecules were inclined to aggregate on copper oxide surface while ethanol molecules were tend to disperse in solution freely. The

adsorption experiments have been conducted and results showed that phenol is more inclined to absorbed on CuO surface (Fig. S7, S8, Text S2). Therefore phenol facilely took up active site on copper oxide surface and inhibited the combination between PCA and the sites. As for ethanol, molecules were not likely to aggregate on surface so there were still enough active sites for persulfate activation. Therefore the main reactive species was activated persulfate.

Table 1. Rate constants of five sulfate and hydroxyl radical scavengers[21] scavengers methanol ethanol tert-butyl alcohol nitrobenzene phenol

k[SO·4-] / M·s-1 6

k[HO· ] / M·s-1 8

3.2×10 7 1.6-7.7×10 5 4-9.1×10 6 < 10 9 8.8×10

9.7×10 9 1.2-2.8×10 6 3.8-7.6×10 9 3.0-3.9×10 9 6.6×10

a

Fig. 3.

b

PCA removal in PS/CuO/PCA system with scavengers ethanol and phenol. (a) no -1

-1

pH adjustment, (b) pH 7. Conditions: [PS]0 = 2.5 mmol·L , [PCA]0 = 0.5 mmol·L , [CuO] = -1

-1

-1

0.5 g·L , [ethanol] = 500 mmol·L , [phenol] = 2.5 mmol·L . a: pH = 7, b: no pH adjustment

3.3.2. Surface activation mechanism verification

Evidences were still needed to support surface activation mechanism. Therefore, The influence of pH and surface copper density on PCA degradation efficiency were investigated. In the study of influence by pH, neutral (initial pH was adjusted to 7) and acidic (no pH adjustment, initial pH was 3) condition was focused on. Basic condition was excluded because base could activate persulfate to generate radicals[13] which would interfere persulfate activation mechanism study. The results were shown in Fig. 4. The pseudo-first-order linear fitting was done for the PCA degradation line. The PCA removal rate and apparent rate constant were 71.5%, 0.0034 min-1 (R2 = 0.995) for pH adjustment and -1

2

64.1%, 0.003 min (R = 0.987) for no pH adjustment (Fig. 4a, S9). According to our previous work[41], more copper ions would dissolved from oxide surface through the reaction between hydrogen ions and copper oxide at pH 3 than pH 7, the result in Fig. 4b supported the conclusion. Copper ions has also been reported to activate persulfate44, the lower degree of fitting for PCA degradation line at pH 3 indicated the evident influence of increasing dissolved copper ion with experiment time pass by, making the curve straight but not concave. So the persulfate activation by copper ions homogeneously and by copper oxide surface heterogeneously were both possible activation pathway. According to the result of copper ion dissolution study during the experiments, less copper ions dissolved when initial pH was 7 than the initial pH was 3. Considering higher PCA degradation efficiency at initial pH 7, we proposed that persulfate activation was mainly achieved by oxide surface [41], while copper ions contributed a little part (as described in Diag. S1). In surface activation scenario, the surface charge was positive when initial pH was neutral or acidic as reported pHzero of copper oxide was 9.5[42, 46]. The pKa of PCA was 4.2, which meant that PCA was

protonated in acidic condition and deprotonated in neutral condition (SI, slide 8). So when initial pH was 7, more deprotonated PCA and persulfate ions tended to be absorbed by positive copper oxide surface and enhanced the degradation (the pH variation profile during the experiment was showed in Fig. S10). In the study of influence by surface copper density, the copper oxide was repeatedly used for three times and study the dissolved copper ion concentration evolution during the reaction. Copper oxide was separated after reaction, first washed by deionized water for three times to remove water soluble materials, second washed by ethanol for three times to remove PCA intermediates deposition on copper oxide surface, last washed by deionized water for three times to remove ethanol and residual intermediates. The washed copper oxide was dried at 108 ℃. Results showed that when the initial pH was 7, PCA removal was decreased from 64.7% to 51.8% then to 40.6%, about 10% PCA removal loss for every run (Fig. 5a). Comparing the dissolved copper ion evolution at first run and third run, we can find significant decreased dissolved copper ion concentration when copper oxide was reused (Fig. 5b). This might be related to decreased surface copper density (refer to the EDS results of reused copper oxide, Table. S2), which would weaken the persulfate activation and PCA degradation. This evidence supported that persulfate was mainly activated by copper oxide surface.

a

b

Fig. 4. Effect of solution pH on the rate of PCA degradation in the systems containing (a), dissolved copper ion concentration evolution in systems with different pH (b). Conditions: [PS]0 = 2.5 mmol·L-1, [PCA]0 = 0.5 mmol·L-1, and [CuO] = 0.5 g·L-1.

a

b

Fig. 5. PCA degradation with reused copper oxide at pH 7 (a), dissolved copper ions concentration evolution in systems with pristine and third reused copper oxide (b). -1 -1 -1 Conditions: [PS]0 = 2.5 mmol·L , [PCA]0 = 0.5 mmol·L , [CuO] = 0.5 g·L . 3.3.3. EPR study. PCA degradation inhibition by addition of ethanol and phenol still caused the suspicion of radicals generation. Electron paramagnetic resonance was an intuitionistic method to detect

radicals. Hydroxyl and sulfate radicals could react with DMPO to form DMPO-OH or DMPO-SO4 adducts which could be detected by electron paramagnetic resonance instrument. The spectrums obtained from EPR were simulated by WinSim package[47-49], the hyperfine coupling constants were measured and radical species were identified (Fig. 6). It showed no radical produced in mixture of water with copper oxide, but sulfate and hydroxyl radical signals were observed in persulfate solution at pH 7 and no pH adjustment (Fig. 6a). When copper oxide was introduced in persulfate solution, signals increased obviously in both neutral and acidic systems (Fig. 6b). Acidic system created a stronger signal than neutral system. Free copper ions has been reported to activate persulfate to produce sulfate radicals which could further form hydroxyl radicals (as signals shown in Fig. 6f) through reaction 51

with water and hydroxyl ions subsequently . Based on the difference of copper ion dissolution between acidic and neutral system (Table. S3), stronger signal could be explained by more copper ions dissolution when related to the result of dissolved copper ion evolution (Fig. 4b). To further investigate the radical generation source, leaching study were conducted. Leaching solution was prepared by filtering the mixture of persulfate solution and copper oxide after shaking for 5 min at 20℃ and reacted with DMPO. Fig. 6c showed that Cu 2+ could activate persulfate to form sulfate and hydroxyl radicals and Fig. 6d verified that more Cu2+ resulted in more radicals. Since heterogeneous activation of persulfate could produce 22,23,36

sulfate and hydroxyl radicals

, Water/PS/CuO/ethanol system and water/PS/CuO/phenol

system were tested by EPR to figure out the contribution of copper oxide surface for radical generation. The signal vanished when phenol was introduced, while the signal wasn’t fully wiped out when ethanol superseded phenol (Fig. 6e). As discussed above, ethanol couldn’t

contact the surface adequately because of the high dielectric constant so the radicals produced from surface couldn’t be quenched completely. The detected radical signal was probably produced from copper oxide surface activated persulfate. While phenol could aggregate on surface, inhibiting the combination between persulfate ions and surface active sites as well as consuming the generated radicals. The formed ethanol radical confirmed the radical generation again. Combined the result in persulfate decomposition study (Fig. 2b) and radical scavenger quenching experiment (Fig. 3a,3b), the generated radicals were not the main oxidative species to degrade PCA. Also, evidence that CuO/PS system showed much weaker 2+

signal but higher PCA degradation rate than Cu /PS system supported the proposal. The non-radical-generated activation of persulfate was dominant process occurred on copper 2+

oxide surface. The detected radicals came from Cu , and copper oxide surface, which could be regarded as “side reactions”.

Fig. 6. EPR signals of target systems. a: blank systems, including mixture of water with copper oxide, persulfate solution at pH 7 and no pH adjustment; b: PS/CuO systems at pH 7 and no pH adjustment; c: CuO/PS system, LS/PS system, LS was leaching solution extracted from mixture of CuO and persulfate solution after being shaken for 1 min, no pH adjustment; d: LS/PS systems at pH 7 and no pH adjustment; e: PS/CuO/ethanol, PS/CuO/phenol -1

-1

2+

systems, [ethanol] = 500 mmol·L , [phenol] = 2.5 mmol·L ; f: CuO/PS system, Cu /PS

2+

system, Cu concentration was equivalent to Cu content in CuO. Conditions: [PS]0 = 2.5 mmol·L-1, [CuO] = 0.5 g·L-1, [Cu2+] = 0.4 g·L-1, room temperature. The positions marked by green circle, blue triangle, red square were assigned to hydroxyl radical (DMPO-OH, aH = N

H

N

59

14.6 G and a = 15.0 G as simulated, a = 14.9 G and a = 14.9 G in literature ), sulfate N

H

H

H

N

radical (DMPO-SO4, a = 13.7 G, aα = 9.9 G, aβ = 1.43 G, aγ = 0.76 G as simulated, a = 13.8 G, aαH = 10.1 G, aβH = 1.38 G, aγH = 0.68 G in literature60) and ethanol redical (DMPO-CHOHCH3, aH = 22.8 G, aN = 15.8 G 61). 3.4.Mechanism interpretations. According to above results and discussion, experiment facts were obtained as follows: 1. persulfate wouldn’t be consumed by copper oxide without electron donor like PCA. 2. CuO/PS/PCA/pH 7 system present lower dissolved copper ion concentration but achieved more PCA removal than CuO/PS/PCA/pH 3 system. 3. the reused copper oxide showed decreased persulfate activation ability and lower dissolved copper ion concentration, 4. ethanol slightly inhibited PCA degradation while phenol showed significant inhibition effect in CuO/PS/PCA system, 5. EPR study had detected the hydroxyl and sulfate radical in CuO/PS/DMPO system and acidic condition generated more radicals than neutral condition, 6. copper ion could activate persulfate to form radicals, 7. leaching solution of CuO/PS/pH 3 system generated more radicals than that of CuO/PS/pH 7 system, 8. phenol could completely wipe out generated radicals but ethanol couldn’t.

Taking all the facts into account, we can concluded that activated persulfate, sulfate radical and hydroxyl radical were all reactive oxygen species (as described in Diag. S2). Activated persulfate was dominant and generated from copper oxide surface heterogeneously. Most persulfate ions connected with copper sites by outer-sphere interaction without forming strong bond or radical generation[42], while persulfate might also oxidized surface Cu(II) to Cu(III) and generate sulfate radical (equation 3). The formed Cu(III) would oxidize adsorbed water into hydroxyl radicals (equation 4)[50]. Duan[22] reported that electrons transferred from absorbed water or hydroxyl groups to nanocarbon then to persulfate, resulting in hydroxyl radical and sulfate ion generation in dimensional structured nanocarbon-persulfate system. Our work proposed that Cu(II)/Cu(III) cycle followed the similar electron transfer path, both in solution

51, 53

and on surface (equation 3, 4, 5, 6). It was reported that Cu(III)

could oxidize organic compounds[51-53] or generate radicals to decompose target pollutants, but our previous work[41] verified that Cu2+/PS system showed limited PCA removal compared with CuO/PS system, indicating that non-radical-generated activation of persulfate was dominant process (equation 1 and 2). The total activation mechanism could be described as follows: ≡CuIIsites + S2O82- → Cu II⋯S2O82-

(1, fast)

≡CuII⋯S2O82-+ PCA → ≡CuIIsites + 2 SO42- + products

(2, fast)

≡CuIIsites + S2O82-(trace) → ≡CuIIIsites + SO42- + SO4∙-

(3)

≡CuIIIsites + H2O → ≡CuIIsites + ∙OH + H+

(4)

Cu + S2O8 → Cu + SO4 + SO4∙

(5)

2+

2-

3+

2-

Cu + H2O → Cu + ∙OH + H 3+

2+

+

-

(6)

SO4∙ + H2O → SO4 + ∙OH + H -

2-

+

(7)

Therefore the main reactive oxygen species was activated persulfate while sulfate and hydroxyl radicals were still generated.

3.5. Applicability of copper oxide activated persulfate in view of effects of bicarbonate and chloride ions In natural water, some co-existing ions such as chloride and bicarbonate ions show a quenching effect to sulfate and hydroxyl radicals[54] and reduce persulfate oxidation capacity. Three systems namely PCA/PS/CuO, PCA/PS/CuO/NaHCO3 and PCA/PS/CuO/NaCl were carried out to compare the effect of these two kinds of ions. The addition of 2.5, 5 and 10 mmol·L-1 sodium chloride showed little effect on PCA degradation rate at both neutral and acidic condition(Fig. 7a, 7b). While in the case of bicarbonate -1

addition, there were some appealing phenomena. 2.5 and 5 mmol·L bicarbonate increased PCA degradation rate before the designed reaction end when initial pH was not adjusted, but finally PCA degradation rate in system without bicarbonate addition would caught up. The PCA degradation line of system without bicarbonate addition intersected with that of system with 10 mmol·L-1 bicarbonate addition and finally exceeded downward. These two findings implied that the increased pH (Fig. S11) after bicarbonate addition would enhance persulfate activation, but the growing dissolved copper ion in system without bicarbonate addition would drive the increasing persulfate activation, and the bicarbonate might show inhibition on surface activation of persulfate (Text S3). And finally PCA degradation rate without

bicarbonate addition caught up and exceeded. In this occasion, slight of bicarbonate would increase the persulfate activation by adjusting the pH while higher bicarbonate would cause a opposite result. When initial pH was adjusted to neutral (before the bicarbonate addition), the addition of bicarbonate would slightly inhibit PCA removal and there was no obvious inhibition difference at various amount of bicarbonate addition (Fig. 7c, 7d), which was probably due to the increased pH up to basic (Fig. S9). The findings of no significant PCA removal inhibition by bicarbonate and chloride ions could be another evidence for non-radical-generated activation of persulfate. Bicarbonate and chloride ions could react with hydroxyl and sulfate radical[6, 55, 56], which results in the competitive reactions against target pollutants in systems where radicals were main oxidative species. The target pollutants degradation rate would decrease if the quenchers exist. According to the results, it is evidenced that the driving force of PCA degradation was activated persulfate on which bicarbonate and chloride ions had no significant effect ( E0 [Cl2·/ Cl-] = 2.09 V [57], E0 ·

[CO3- /CO3 ] = 1.59 V [58], E [S2O8 /SO4 ] = 2.01 V, the low redox potential gap obstructs 2-

0

2-

2-

the oxidation of chloride and bicarbonate by activated persulfate, and the confine of persulfate on surface might make it more selective for target reactant molecule orientations. The redox potential for Cu(III)/Cu(II) in solid and solution states are 2.3V and 1.57V [42, 43] respectively. Compared with E0 [Cl2 ·/ Cl-] = 2.09 V [57], E0 [CO3- ·/CO32- ] = 1.59 V, the E0 [Cu(III)/Cu(II)] is proper to form the reaction between Cu(III) and CO32-, but since the generated Cu(III) oxidized from Cu(III) is very few, so the effect of reaction could be neglected). The result demonstrated the feasibility of copper oxide activated persulfate in

ISCO application since ubiquitous chloride and bicarbonate ions showed no adverse effects. Activated persulfate was active enough to oxidize PCA.

a

b

c

d

Fig. 7. Effect of chloride and bicarbonate ions on the rate of PCA degradation. (a) Chloride ion, in CuO/PS/PCA/no pH adjustment system, (b) Chloride ion, in CuO/PS/PCA/pH 7 system, (c) bicarbonate ion, in CuO/PS/PCA/no pH adjustment system, (d) bicarbonate ion, in CuO/PS/PCA/pH 7 system, Conditions: [PS]0 = 2.5 mmol·L-1, [PCA]0 = 0.5 mmol·L-1, [CuO] = 0.5 g·L-1, [Cl-] = 2.5, 5, 10 mmol·L-1, [HCO3-] = 2.5, 5, 10 mmol·L-1

4. Conclusion The monoclinic rough-surface copper oxide was prepared in lab by cupric nitrate calcination. As a heterogeneous activator, copper oxide could activate persulfate to a more active state, which oxidized PCA and achieved 71.5% removal rate in aqueous after 5 hours. Compared with PS/CuO and PS/PCA systems, the significant decomposition of persulfate in

PS/CuO/PS system led to the conclusion that persulfate was activated without radicals generation. Sulfate and hydroxyl radical scavenger ethanol only caused 19.2% and 13.7% decrease of PCA removal rate in neutral and acidic condition respectively, indicating not radicals but partially activated persulfate was dominant reactive oxygen species generated from persulfate activation. While phenol caused 52.6% and 48.9% decrease of PCA removal rate compared with ethanol. The significant inhibition effect was due to phenol aggregation on copper oxide surface obstructing persulfate activation. The fact that neutral system containing less dissolved copper ions was more suitable than acidic system for persulfate activation and resulted in higher PCA removal rate indicated that surface activation was dominant and copper ion activation was minor, which could explained phenol inhibition effect. EPR results confirmed the existence of trace sulfate and hydroxyl radicals generated from persulfate activation by dissolved copper ions and a few active sites on copper oxide surface. Considering all the experiment facts, it is proposed that activated persulfate was dominant reactive species, along with minor sulfate and hydroxyl radicals generated in -1

copper oxide activated persulfate system. 2.5, 5 and 10 mmol·L chloride ions exerted no negative effect to PCA degradation. While 2.5, 5, 10 mmol·L-1 bicarbonate ions promoted PCA degradation in acidic initial condition and showed slight inhibition effect in neutral condition. In conclusion, copper oxide activated persulfate with trace radical generation, which could be a potential in situ chemical oxidation method for refractory organic compounds, since ubiquitous chloride and bicarbonate ions won’t show adverse effects and the method would avoid persulfate consumption due to radical quenching effects caused by non-pollutant reductive materials, while the detection of high copper ion concentration should

be considered as a barrier of application which we are trying to break by dragging or trapping Cu atom more tightly.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51572089), the National Natural Science Foundation of Guangdong Province, China (2015A030313232), the Foundation of Science and Technology Planning Project of Guangdong Province (No.2016A050502007), the Research Funds of State Key Lab of Subtropical Building Science, South China University of Technology (2015ZB25) and the Funds of the State Key Laboratory of Pulp and Paper Engineering, China (201477).

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Highlights







Copper oxide showed an excellent performance on persulfate activation to eliminate p-chloroaniline Chloride and bicarbonate ions had no negative effect on p-chloroaniline degradation by copper oxide activated persulfate Partially activated persulfate was dominant reactive oxygen species in persulfate activation by copper oxide Trace sulfate and hydroxyl radicals were generated in copper oxide-persulfate system