Cobalt oxyhydroxide as an efficient heterogeneous catalyst of peroxymonosulfate activation for oil-contaminated soil remediation

Science of the Total Environment 680 (2019) 61–69

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Cobalt oxyhydroxide as an efficient heterogeneous catalyst of peroxymonosulfate activation for oil-contaminated soil remediation Cong Lyu a,b,⁎, Dan He a,b, Yuming Chang a,b, Qihui Zhang a,b, Fang Wen c, Xiansheng Wang a,b a b c

Key Lab of Groundwater Resources and Environment, Ministry of Education, Jilin University, Changchun 130026, PR China Jilin Provincial Key Laboratory of Water Resources and Environment, Jilin University, Changchun 130026, PR China Xinjiang Key Laboratory for Environmental pollution Monitoring and Risk Warning, Xinjiang Academy of Environmental Protection Science, Urumqi, Xinjiang 830011, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• CoOOH was an efficient peroxymonosulfate activator. • SO4•−, O2•− and 1O2 were involved in the PMS/CoOOH system. • CoOOH displayed good stability during the catalytic oxidation. • The PMS/CoOOH system was a potential alternative for soil remediation.

a r t i c l e

i n f o

Article history: Received 3 February 2019 Received in revised form 10 April 2019 Accepted 21 April 2019 Available online 6 May 2019 Editor: Daniel CW Tsang Keywords: Cobalt oxyhydroxide Sulfate radical Singlet oxygen Superoxide radical Peroxymonosulfate

⁎ Corresponding author. E-mail address: [email protected] (C. Lyu).

https://doi.org/10.1016/j.scitotenv.2019.04.324 0048-9697/© 2019 Elsevier B.V. All rights reserved.

a b s t r a c t The persulfate/Fe2+ system has been proposed for the chemical oxidation for soil remediation, however, the homogeneous iron catalyst was hard to reuse which limited the further application. Cobalt oxyhydroxide (CoOOH) existed as a mineral in nature, which was environmentally friendly. Thus, in this study, CoOOH was selected as an efficient heterogeneous catalyst for peroxymonosulfate (PMS) activation to remediate oil contaminated soil by chemical oxidized reaction. 88.3% of oil at the initial concentration of 78–99 mg/kg can be removed within 24 h under the conditions of 1.0 g/L CoOOH and 0.1 M PMS at room temperature. The residual oil content was approximately 11.5 mg/kg which was lower than the standard of petroleum hydrocarbons for residential land (30 mg/kg), published by the Canadian Council of Ministers of the Environment (CCME). Specifically, the PMS/ CoOOH system had a relatively high apparent reaction rate constant (0.3078 h−1), which was approximately twice that of the PS/Fe2+ system (0.1601 h−1). Furthermore, multiple radicals and reactive oxygen species (ROS), such as SO4•−, O2•− and 1O2, were involved in the oil removal oxidation reaction. Moreover, 73% total organic carbon (TOC) had been removed after the reaction. The findings of this study suggested that the oilcontaminated soil and CoOOH could both be recycled after remediation using the PMS/CoOOH system. In summary, the results indicated that CoOOH is a promising heterogeneous catalyst, and the PMS/CoOOH system could be considered as a feasible alternative to the PS/Fe2+ system for the remediation of oil-contaminated soil. © 2019 Elsevier B.V. All rights reserved.

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1. Introduction Widespread oil spills have led to the rapid expansion of areas of oilcontaminated soil (Pinto et al., 2018; Varjani et al., 2015), which have resulted in environmental contamination and potential human health risks (Varjani, 2017). Bioremediation has been applied for the remediation of oil-contaminated soil, nonetheless, its use is restricted as a result of the poor biodiversity and great dependence on the external parameters (Venosa and Zhu, 2003; Ron and Rosenberg, 2014). Therefore, it is essential to develop innovative technologies for pollutant remediation (Bachmann et al., 2014). As a potent soil remediation option, chemical oxidation has exhibited great efficiency in oil removal (Usman et al., 2018). Specifically, the sulfate radical-based chemical oxidation (SO4•−, E0 = 2.5–3.1 V) has attracted wide attention due to its independence from pH as well as the superior selective oxidation (Li and Jiang, 2016; Liang et al., 2004). SO4•− can be generated through activating persulfate (PS) or peroxymonosulfate (PMS) under ultraviolet light and through the transition metal in soil (Yen et al., 2011; Bandala et al., 2008). Among them, the activation of transition metal is favored as a soil remediation method, which can be attributed to its simple operation, rapid result and low operational costs (Lim et al., 2016; Li et al., 2016a). Recently, in comparison with PS, PMS has attracted increasing attention, which can be attributable to its unsymmetrical chemical structure that can accelerate the reaction rate (Flanagan et al., 1984; Dugan et al., 2010). Anipsitakis and Dionysiou suggested that Co2+ was superior to the other transition metals for PMS activation (Anipsitakis and Dionysiou, 2004). The PMS/Co2+ system was successfully tested for soil remediation (Sihyun et al., 2009); however, the discharge of toxic cobalt ions is problematic, and therefore research interest has focused on the investigation of other cobalt-based heterogeneous catalysts with stable structure, environmentally friendly characteristics and excellent catalytic ability to activate PMS (Deng et al., 2017a). Among the cobalt containing materials, cobalt oxyhydroxide (CoOOH) has attracted great interests as an alternative heterogeneous catalyst for PMS activation, because of its abundant and outstanding characteristics (Elizarova et al., 2000), including potent electron transfer rate, high surface hydroxyl sites, multiplied surface active sites, capability of various phase reactions, and extensive presence in Katanga Copperbelt (Decrée et al., 2015). Importantly, CoOOH is a naturally occurring mineral, it is a stable and multifunctional material, which has been applied as a CO sensor (Wu et al., 2006), precursor for electrode materials in energy storage and high performance super capacitor (Hou et al., 2013a; Fu et al., 2007; Dhawale et al., 2015). This study aimed to assess oil removal by PMS with CoOOH as the heterogeneous catalyst, and the PMS/CoOOH system was applied for oilcontaminated soil remediation. Therefore, this study mainly aimed: (i) to establish an efficient heterogeneous catalytic PMS/CoOOH system for soil remediation; (ii) to evaluate the efficiency and recyclability of the PMS/CoOOH system for oil removal from soil; (iii) to identify the underlying mechanisms of oil removal from soil using the PMS/CoOOH system through quenching experiment, X-ray photoelectron spectroscopy (XPS), electron spin-resonance spectroscopy (ESR), total organic carbon (TOC), X-Ray fluorescence (XRF) and gas chromatographymass spectrometer (GC–MS) analysis. 2. Materials and methods 2.1. Chemicals and materials Oil-contaminated soil was collected from Karamay oil-contaminated field, Xinjiang province. The oil in the oil-contaminated soil was the target pollution. Potassium peroxymonosulfate (KHSO 5 · 0.5KHSO4·0.5K2SO4), were used as oxidant. Besides, Cobalt nitrate hexahydrate (Co(NO3)2·6H2O), 30% hydrogen peroxide (H2O2), sodium hydroxide, sodium sulfate (Na2S2O8), ferrous sulfate (FeSO4·7H2O) and

nano zero-valent iron were supplied by Aladdin Chemistry Co., Ltd. (Shanghai, China). Tert-butyl alcohol (TBA), ethanol (EtOH), pbenzoquinone (p-BQ) and sodium azide (NaN3) were purchased from Tianjin Kermel Chemical Reagents Company (Tianjin, China). Carbon tetrachloride (CCl4) and magnesium silicate (Mg3Si4O10 (OH)2) were obtained from Tianjin Aoran Fine Chemical Research Institute. All above solvents and chemicals used in the experiments were analytical grade. 2.2. Pretreatment of oil-contaminated soil The characteristics of soil samples are shown in Table S1. More than 36.01% particle sizes were over 50 μm. The initial oil concentration in the contaminated soil was 78–99 mg/kg (dry soil). Coarse gravel and stones were removed from all soil samples, and then the samples were sieved by a 5 mm sieve. After that, they were stored at 4 °C fridge. 2.3. Synthesis of CoOOH In this study, CoOOH had been synthesized according to the modified chemical precipitation method (Hou et al., 2013b). Briefly, 1 mol/L NaOH solution was added into 0.05 mol/L Co(NO3)2 solution until the color of the mixed solution had turned into dark blue. Subsequently, 30% excess H2O2 was dropwise added into the beaker, and the final solution was filtered, washed for at least three times and dried at 60 °C. 2.4. Experimental The activities of CoOOH for oil removal in the catalytic PMS oxidation system under different conditions were evaluated by a series of batch experiments. Then, the optimal parameters were determined after the oil removal performance test. Apart from the heat treatment experiments, all the reactions were conducted in sealed glass bottles (100 mL) at 200 rpm and at room temperature (23 ± 2 °C). Prior to the chemical oxidation reactions, 5 g of oil-contaminated soil was added in 50 mL deionized water in each vial. Then, 10 mL of PMS solution and synthesized CoOOH were added to the vial. Slurry samples at different time intervals were extracted by CCl4 through vibration for 0.5 h–1 h. Afterwards, amount of anhydrous Na2S2O8 was added to the extracted liquid. Then, the oil quantification was based on the method of infrared spectrophotometry. The method of oil quantification was based on the American standard of “Total Recoverable Petroleum Hydrocarbons by Infrared Spectrophotometry” (EPA 8440). As for the recycle experiment, catalyst (CoOOH) could be separated out by sieving for consecutive characterization. To evaluate the potential application of recycled material, an experiment was conducted using an amount of contaminated soil consistent with the initial oil concentration. The species of free radicals were identified by adding the probe materials of TBA, EtOH, p-BQ and NaN3. All batch experiments were conducted with two parallel samples. A first-order kinetics pattern was applied to investigate the kinetics of oil removal. 2.5. Characterization and analysis The crystal structure of CoOOH was determined through X-Ray diffraction analysis (XRD) (XD-3, Shimadzu Corporation, Japan). In addition, the surface morphology of CoOOH was observed under a highly qualitative scanning electron microscope (SEM) (JSM-6700 F, JEOL Ltd., Japan). The elemental composition of CoOOH was determined by energy dispersive spectrometer (EDS) (X-Max 50, Oxford Instrument, U.K.). Besides, the variations in the chemical states of cobalt and oxygen elements were analyzed using X-ray photoelectron spectroscopy (XPS) (ESCALAB 250, Thermo Corporation, USA). Further, the functional groups of CoOOH were further characterized by Fourier transformed infrared spectrometer (FTIR) (8400S, Shimadzu Corporation, Japan), whereas the BET surface areas and pore size of CoOOH particles were

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characterized using the specific surface analyzer (Quantachrome NOVA1000, USA). To analyze the particle size in the soil samples, size distribution was carried out in the samples using the Laser particle size distribution instrument (Bettersize 2000, Dandong, China). Oil concentration was analyzed through the infrared spectroscopy technique (JDS-108U+, JLBG, China). To determine the mineralization efficiency, total organic carbon (TOC) content was measured using a Jena TOC analyzer (SSM-5000A). 1.0 M hydrochloric acid was added into the dried soil samples as pretreatment before TOC analysis to minimize the effect of inorganic matter on the data. Typically, the TOC value was plotted based on the measured value of total organic matter subtracted by that of the inorganic and organic matters. For element analysis of the unknown mineral and the relative element concentration in soil samples, X-Ray fluorescence (XRF) (AXIOS, Panalytical B.V., Netherlands) had been employed in this study. Free metal ions were determined through the inductively coupled plasma mass spectrometry (ICP-MS) (Agilent 7500C, USA). Besides, electron spin resonance (ESR) experiments had also been performed using the spectrometer (JES FA200, Jeol, Japan), with DMPO and TEMP as the spin-trapping agents. Moreover, To further clarify the material changes of oil in the soil samples after the removal reaction, the material compositions of oil in the contaminated soil and remediated soil were analyzed according to the scan mode of gas chromatography-mass spectrometer (GC–MS) (Trace ISQ, Thermo Scientific, USA).

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which could be indexed to the (003), (101), (012), (015) and (110) planes of CoOOH (JCPDS Card No. 73-1213), respectively. These findings indicated the stable crystal form of the CoOOH prepared through the chemical precipitation method. To further observe the micromorphology of the as-synthesized CoOOH, SEM analysis was conducted. The CoOOH nanoparticles had been formed uniformly as the nano-size hexagonal rings, which had relatively good dispersion and high specific surface area (Fig. 1B). EDS analyses (Fig. 1C) illustrated that the elemental composition included Co, O and H, which was in good agreement with the expected stoichiometry. The survey spectrum of XPS in Fig. S1B indicated the presence of the elements Co and O on the catalyst surface. Moreover, the FTIR pattern showed that the peak at 1639 cm−1 was characteristic of the Co\\O double bond and the Co\\OH group below 1000 cm−1 in the crystal structure of CoOOH (Li et al., 2015a). Thus, it could be confirmed that the as-synthesized catalyst in this study was pure CoOOH. To further understand the particle structure of CoOOH, N 2 adsorption/desorption measurements were performed. The specific surface area, the pore sizes and their distribution of CoOOH and the results were depicted in Fig. S2. The results showed that CoOOH of 27.57 nm average pore size with good surface area (50.33 m2/g) provided sufficient specific surface-active sites to act as a PMS activator. 3.2. Catalytic activity of CoOOH during PMS activation for oil removal from soil

3. Results and discussion 3.1. Characterization of CoOOH The XRD pattern (Fig. 1A) of the CoOOH particles showed several diffraction peaks at 2θ = 20.242, 37.015, 38.932, 50.670 and 65.416°,

The catalytic activity of CoOOH during PMS activation for oil removal was investigated. A series of different chemical oxidation processes was carried out to select the most efficient oxidant and catalyst for oil removal from soil. The removal kinetics was further investigated through

Fig. 1. XRD pattern of CoOOH (A); SEM (B) EDS (C) FT-IR (D) images of CoOOH.

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fitting the experimental data into the Langmuir-Hinshelwood model (Fig. 2B) which followed the fitting first-oider kinetics shown as Eq. (1). Ct ¼ C0 e−kt

ð1Þ

First, comparison of the PMS/CoOOH system (0.3078 h−1) and PS/ CoOOH system (0.1841 h−1) indicated that CoOOH as heterogeneous catalyst was more efficient for PMS activation than that for PS. This result was attributed to the asymmetric structure of PMS (Ghanbari and Moradi, 2017). In contrast, comparison of the PS/CoOOH system and the PS/Fe0 system showed that the ability of CoOOH catalyzed PS was lower than that catalyzed by Fe0, it turned out that the ability of CoOOH catalyzed PS was lower than that of Fe0. Therefore, PMS was the proper and efficient oxidant catalyzed by CoOOH. Second, cobalt-based heterogeneous catalyst such as Co3O4 and CoFe2O4 were compared as reference catalysts to evaluate the catalyst catalytic activity of CoOOH for PMS activation. The CoOOH/PMS system possessed a high apparent reaction rate constant, which was higher than that in Co3O4/PMS (0.2020 h−1) and CoFe2O4/PMS system (0.2437 h−1), respectively (Fig. S2). Moreover, CoOOH had much larger BET surface area (50.33 m2/g) than that of Co3O4 (18.00 m2/g) and CoFe2O4 (32.05 m2/g), resulting in good dispersion for interfacial reaction (Chen et al., 2018a; Chen et al., 2018b; Yang et al., 2018; Chen et al., 2018c). To further evaluate the oil removal efficiency of PMS/CoOOH system, it was compared with three other sulfate-based chemical oxidation technologies for the remediation of oil-contaminated soil (PS/Fe2+, PS/Fe0, PMS/Co2+) (Yen et al., 2011; Desalegn et al., 2018; Do et al., 2009). The oil removal rate followed the order of PMS/CoOOH N PMS/Co2+ N PS/Fe0 N PS/Fe2+, indicating that the PMS/CoOOH system possessed the most impressive capability for oil removal from soil, which was attributed to the outstanding electron transfer ability of CoOOH. Above all, the PMS/CoOOH system displayed the highest efficiency among the 7 oxidation reactions tested. The results showed that the PMS/CoOOH system could be a promising alternative process for oil removal from soil. 3.2.1. Effect of temperature on the oil removal from soil Oil removal during a 12-h reaction period at temperatures of 10 °C, 25 °C, 35 °C and 50 °C was 80%, 85%, 87% and 90%, respectively (Fig. 3A). The reaction kinetics of the PMS/CoOOH system followed first order kinetics and the k value of oil removal gradually increased from 0.2856 h−1 to 0.3682 h−1 with increasing temperature. The increasing temperature corresponds to higher reaction rate which might because higher temperature promotes the movement between molecules (Zhang et al., 2019). Thus, the increasing temperature promoted

the contact between ROS and oil on the surface of CoOOH than that in room temperature. Moreover, higher temperature not only promotes the PMS pyrolysis via rupture of the O\\S\\O bond but also enhances the dynamics of adsorption and diffusion of oil/PMS on CoOOH which related to the high electron transfer ability of CoOOH. CoOOH also provides sufficient surface area and many surface hydroxyl sites for catalytic reaction. The temperature dependency of kt was further evaluated by using the Arrhenius equation: ln kt = −Ea/RT + lnA, where A was the pre-exponential factor, Ea (kJ/mol) was the activation energy, R was the universal gas constant and T was the absolute temperature (K), the evaluated active energy was 4.73 kJ/mol. The activation energy of the PMS/CoOOH system was much lower than of PMS/Co2+ system (56.06 kJ/mol), PS/Fe0 system (52.94 kJ/mol) and PS/Fe2+ system (61.86 kJ/mol) (Chen et al., 2019; Romero et al., 2010; Hussain et al., 2017). The extremely low activation energy might confirm the high catalytic reactivity of the CoOOH reaction.

3.2.2. Effects of PMS and CoOOH dosage on oil removal from soil Oil removal was dependent on the ratio of PMS concentration (Fig. 3C). Notably, the oil removal rate increased from 8.3% to 90% when the initial PMS concentration increased from 0 to 0.15 M in 12 h. Correspondingly, the rate constants (k) were 0.0053 h−1, 0.1581 h−1, 0.1845 h−1, 0.3078 h−1 and 0.4211 h−1, respectively, indicating that higher PMS concentrations would result in higher rates of oil removal. However, excessive PMS might be a limiting factor for the efficiency of multiple radicals-based processes (Tao et al., 2012). Based on the above discussion, 0.1 M PMS was selected as the optimal PMS concentration. CoOOH could not achieve any oil removal in the absence of PMS (Fig. 3E), and CoOOH did not contribute to simple physical adsorption. At the CoOOH concentration of 0.1 g/L, 60.8% of oil could be removed in 5 h, while 84.6% was removed at the CoOOH concentration of 1.5 g/L. Correspondingly, the kinetic rate constant also increased from 0.1853 h−1 to 0.3428 h−1, revealing that oil removal could be enhanced with increases in CoOOH concentration. However, the cost of CoOOH over 1.0 g/L was too high to be further applied, so 1.0 g/L was selected as the optimal CoOOH concentration.

3.2.3. Effect of initial oil concentration on the oil removal from soil The absolute quantity of oxidized oil increased from 14.2 to 72.4 mg/kg when the initial oil concentration increased from 17 to 89 mg/kg over 12 h (Fig. S4). The results indicated that increasing the initial oil content would result in more frequent contact between oil and oxidants leading to higher oil removal rate.

Fig. 2. Oil removal under different oxidation processes (A) and the reaction rate constant (B) with different oxidation processes. Conditions: Oil content = 78–99 mg/kg, [PS] = [PMS] = 0.1 M; [CoOOH] = [Co2+] = [Fe2+] = 0.01 M = [Fe0] = 0.01 M; T = 23 ± 2 °C. All samples were in duplicates. All error ranges were b10%.

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Fig. 3. Effect of temperature, PMS concentration, CoOOH dosage and oil content (A, C, E) on the removal. Effect of temperature, PMS concentration, CoOOH dosage and oil content (B, D, F) on the reaction of rate constant. Conditions: Oil content = 78–99 mg/kg, [PMS] = 0–0.15 M, [CoOOH] = 0–0.15 g/L, T = 10–50 °C. All samples were in duplicates. All error ranges were b10%.

3.3. Stability and recyclability of CoOOH The stability and reusability of CoOOH was explored to illustrate the potential for practical application. Leaching of cobalt ions in the PMS/ CoOOH system was shown in Fig. S5. Less than 0.7 mg/L cobalt ions leached after 24-h reaction, which was lower than the Canadian water quality guidelines for the protection of livestock (1.0 mg/L) (CCME (Canadian Council of Ministers of the Environment), 1987). The recycling experiment was illustrated in Fig. S5. Although the removal rates decreased after four round recycle, the oil contents were all lower than the Canadian standard of petroleum hydrocarbons for residential land (30 mg/kg), published by Canadian Council of Ministers of the Environment (CCME, (Canadian Council of Ministers of the Environment), 2008). This might be explained in several ways. First, the adsorbed intermediates during oil removal might cover the CoOOH surface. Second, cobalt leaching is inevitable under acidic conditions. In terms of the CoOOH in the removal cycles, when integrated

with the XRD and SEM patterns shown in Fig. S6, it could be concluded that CoOOH exhibited good stability and recyclability for PMS activation for oil removal from soil. XRF analysis (Fig. S7) determined the elemental content of contaminated soil and remediated soil. The results implied that the residual elements in soil after the application of the PMS/CoOOH system were mostly beneficial metals, which had little influence on soil reuse. Moreover, CoOOH, also known as heterogenite, can be considered as residual deposits in nature, therefore, the remaining CoOOH in soil was stable. In summary, CoOOH can be classified as an environmentally-friendly catalyst. 3.4. Mechanisms of oil removal from soil by the PMS/CoOOH system 3.4.1. Mechanisms of PMS activation catalyzed by CoOOH In a heterogeneous system, the catalyst surface plays a vital part, because the reactions take place on the surface (Du et al., 2017). XPS

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Fig. 4. XPS spectra of CoOOH before (A) and after reaction (B).

analysis of CoOOH before and after use was conducted to elucidate the surface activation mechanism. The Co 2p3/2 peak can be divided into two peaks with binding energy positioned at 780.0 eV and 781.1 eV (Fig. 4A), with relative surface contents of 39.1% and 60.9%, respectively, which could be assigned to Co2+ in the tetrahedral sites and Co3+ in the octahedral sites, respectively (Deng et al., 2017b). After use, the relative surface content of Co2+ and Co3+ changed to 43.5% and 56.5%, respectively. The cycle of Co2+ and Co3+ was related to the donation of electrons from Co2+ and the transformation to Co3+ during catalytic oxidation (Deng et al., 2019).

In the case of O 1 s spectra (Fig. 4B), three oxygen species was achieved by convoluting the asymmetrical signal into the absorbed H2O, absorbed oxygen (O2)/hydroxyl groups (OH−) and superoxygen − species (O2− 2 /O ) (Zhu et al., 2018). The peaks at 532.8 eV, 531.4 eV and 530.2 eV could be assigned to absorbed H2O, metal hydroxide (Co-OH) and superoxygen species, respectively. By comparing the data before and after use, the relative intensity of surface absorbed H2O decreased from 34.4% to 10.1%, indicating that H2O was involved in the oxidation reaction. More importantly, the amount of absorbed oxygen increased from 23.5% to 54.7% with the formation of Co- OH groups

Fig. 5. Effects of radical scavengers on the oil removal for identification of reactive oxygen species in PMS/CoOOH system (A). ESR spectra using DMPO (B, D) and TEMP (C) were spintrapping regents. Conditions: Oil content = 78–99 mg/kg, [PMS] = 0.1 M, [CoOOH] = 1.0 g/L, T = 23 ± 2 °C, [EtOH (TBA)]/[PMS] = 50/1, [p-BQ (NaN3)]/[PMS] = 50/1.

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(CoOH+), which could be the most active oxygen species as well as the most effective species to activate PMS during the surface catalytic reac− tion (Song et al., 2018). Furthermore, the related amount of O2− 2 /O species on the used CoOOH surface declined to 6.9%, when compared with the fresh catalyst, further demonstrating that surface oxygen vacancies facilitated the chemical bonding with PMS (Chao et al., 2016). Simultaneously, the results indicated that CoOOH had sufficient surface oxygen vacancies for the catalyst reaction. The above findings demonstrate why CoOOH exhibited better catalytic activity than conventional catalysts for PMS activation and correspondingly contributed to a higher catalytic decontamination. On the basis of the above analysis, the heterogeneous catalytic oxidation that occurs over the surface of CoOOH can be explained in the following steps (Eqs. (2)–(5)). ☰Co3þ þ HSO5 − →☰Co2þ þ SO5 ˙− þ Hþ

ð2Þ

☰Co2þ þ HSO5 − →☰Co3þ þ SO4 ˙− þ OH−

ð3Þ

☰Co2þ þ H2 O→☰CoOHþ þ Hþ

ð4Þ

☰CoOHþ þ HSO5 − →☰CoOþ þ SO4 ˙− þ H2 O

ð5Þ

2+

3+

In summary, the rapid redox cycle of Co /Co and formation of CoOH+ improved the continuous generation of ROS and ensured the highly efficient process of heterogeneous catalytic oxidation and PMS activation. 3.4.2. Identification of ROS in the PMS/CoOOH system To further identify ROS in the PMS/CoOOH system, quenching experiments were performed using alcohol (TBA) and ethanol (EtOH), pbenzoquinone (p-BQ) and sodium azide (NaN3). The reaction rate constants of EtOH with hydroxyl radical (HO•) and sulfate radical (SO4•−) were 1.6–7.7 × 107 M−1·s−1 and 1.2–2.8 × 109 M−1·s−1 respectively , (Liu et al., 2011). The reaction rate constants of TBA with HO• and SO4•− were 3.8–7.6 × 108 and 4–9.1 × 105 M−1·s−1, respectively (Liang and Su, 2009). The reaction rate constants of p-BQ with superoxide (O2•−) were 0.9–1.0 × 109 M−1·s−1 and NaN3 exhibited a higher reaction rate with singlet oxygen (1O2) (Oliveira et al., 2010). The addition of EtOH dramatically slowed down the oil removal rate, whereas the addition of TBA barely impeded oil removal (Fig. 5A). When compared with EtOH, p-BQ was more efficient at impeding oil removal. Oil removal efficiencies of b50% were achieved in 12 h, suggesting that O2•− was also present in the PMS/CoOOH system. Interestingly, oil removal was dramatically inhibited upon the addition of NaN3 into the reaction, with oil removal efficiency of only 10% in 12 h, which could be explained by the fact that 1O2 showed high selectivity towards oxidizing electronrich compounds (Kellogg and Fridovich, 1975). More importantly, the quenching tests revealed that multiple radicals and ROS such as SO4•−, O2•− and 1O2 were involved, indicating the occurrence of both radical and non-radical processes in the PMS/CoOOH system. To further confirm this finding, ESR was applied to identify the generated ROS. DMPO was employed as a spin trapping agent to identify OH• and SO4•−, through the formation of spin-adducts DMPO-OH and DMPO-SO4 respectively (Li et al., 2016b; Guodong et al., 2013). The ESR spectra were obtained from the DMPO experiment after 1, 3 and 5 min reaction times (Fig. 5B). The strong ESR signal with a peak height ratio of 1:2:2:1 is most likely to be DMPO-OH. Interestingly, DMPO-SO4 was detected as a low peak. However, from the quenching observations, OH• was not detected in the reaction. This might be because DMPO-SO4 was short-lived and difficult to detect. The ESR spectra between 1-min and 5-min reactions were compared, which suggested that more DMPO-SO4 was generated and converted into DMPO-OH rapidly, and such results were consistent with the fact that the peak of DMPO-OH became higher with increase in reaction time (Guodong et al., 2013; Li et al., 2019; Li, 2015). The spectra in Fig. 5C and D were consistent

Fig. 6. TOC removal rate of oil between the contaminated soil and remediated soil. Contaminated soil in TOC removal experiment was set as control sample. Conditions: Oil content = 78–99 mg/kg, [PMS] = 0.1 M, [CoOOH] = 1.0 g/L, T = 23 ± 2 °C.

with the results of O2•− and 1O2 quenching experiments. On the basis of prior research, the ROS might have been generated according to the following reactions (Eqs. (6)–(11)) (Ball and Edwards, 1956; Burns, 2012; Li et al., 2015b): HSO5 − þ SO5 2− →HSO4 − þ SO4 2− þ 1 O2

ð6Þ

HSO5 − þ H2 O→HSO4 − þ H2 O2

ð7Þ

SO5 2− þ H2 O→SO4 2− þ H2 O2

ð8Þ

H2 O2 →Hþ þ HO2 ˙−

ð9Þ

HO2 ˙− →Hþ þ O2 ˙−

ð10Þ

2O2 ˙− þ 2Hþ →1 O2 þ H2 O2

ð11Þ

According to the above experimental results, the activation mechanism of PMS on CoOOH can be depicted in Fig. 7 and described by Eqs. (2)−(11). First, H2O molecules were physically adsorbed on part of the metal ions that serve as Lewis sites to generate CoOH+ complexes on the catalyst surface (Tan et al., 2017). Once PMS was added, HSO− 5 ions were formed from PMS being activated by cobalt species (CoOH+). Meanwhile, the cycle of Co2+/Co3+ was generated on the surface oxygen

Fig. 7. Illustration of possible remediation mechanisms of oil-contaminated soil under PMS/CoOOH system.

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Table 1 Comparison of organic compounds between contaminated soil and remediated soil. Conditions: Contaminated oil content = 78–99 mg/kg, [PMS] = 0.1 M, [CoOOH] = 1.0 g/L, T = 23 ± 2 °C. Organic compounds Contaminated soil

Remediated soil

C9H10O C11H10O6 C15H13N C17H14O4 C20H12 C26H42O4 C2H8O2Si

C10H6ClNO2 C12H17NO2 C15H24O2Si C18H12S C21H16O C30H50O5 C6H5Cl

C10H8 C13H22OSi2 C16H10 C19H34O6 C22H14 C30H52O2 C6H6

C11H11Cl2N2 C14H20N2O4S C16H10O C19H21N3O4 C24H36O2Si2 C30H38O12 C8H9NO2

vacancies of CoOOH. The cycling of Co2+/Co3+ was attributed to the transformation of electrons from Co2+ to Co3+ during the catalytic oxidation. Simultaneously, ROS such as SO4•−, O2•− and 1O2 radicals were continuously generated by activated PMS. These ROS could effectively remove oil. 3.4.3. Mechanism of ROS attacking oil content in the PMS/CoOOH system The results of TOC removal using the PMS/CoOOH system were shown in Fig. 6, which revealed the soil mineralization degree in the PMS/CoOOH system. After 12 h of reaction time, 73% of TOC had been removed, among which, most of the TOC (about 65.4%) had been removed within the first 5 h of reaction time. The remaining 27% of TOC might be explained as the byproducts of incomplete oil oxidation and residual soil organic matter. It indicated that ROS attack oil as well as soil organic matter, resulting in high TOC removal. Possible oil removal products by the PMS/CoOOH system were proposed based on the results from scan mode of GC–MS (Figs. S8 and S9). The comparison between the initial soil and soil after remediation is presented in Table 1. The oil was consisted of large molecular hydrocarbons and small molecular hydrocarbons. After chemical oxidation, the oil was degraded to smaller molecule hydrocarbons and CO2 and H2O. Thus, these results showed that the long chain carbon molecules had been degraded into shorter ones, which was consistent with the TOC removal data. 4. Conclusion In this study, CoOOH was proposed as an efficient heterogeneous catalyst for PMS activation for the remediation of oil-contaminated soil. The PMS/CoOOH system showed higher efficiency for oil removal than that of the PS/Fe2+ system, which was attributed to the outstanding catalytic performance of CoOOH as PMS activator. Typically, 88.3% of oil at the initial concentration of 78–99 mg/kg could be removed within 24 h under the conditions of 1.0 g/L CoOOH and 0.1 M PMS. Furthermore, multiple radicals and ROS, such as SO4•−, O2•− and 1O2, contributed to the oxidation reaction. More importantly, CoOOH as PMS activator displayed good stability and recyclability, indicating that it is an environmentally friendly catalyst. Therefore, the PMS/CoOOH system could be considered as an alternative for oil-contaminated soil remediation, exhibiting substantial potential for the remediation of oil-contaminated soil. Acknowledgement The present work was funded by Jilin Provincial Science & Technology Department (Grant No. 20180101081JC), the Science and Technology Support Program for Xinjiang Uygur Autonomous Region (Grant No. 2016E02102), the Science and Technology Project of the Education Department of Jilin Province (Grant No. JJKH20190125KJ) and 111 Project (B16020). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2019.04.324.

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