Copper oxide (CuO) was prepared through a facile one-step hydrothermal method and used as an activator of peroxymonosulfate (PMS) for the degradation of azo dye Acid Orange 7 (AO7), as a model organic pollutant. The effects of several parameters on the decolorization efficiency on AO7 were investigated. The degradation kinetic characteristics were studied by investigating the effect of different operational parameters including CuO dosage, pH and initial AO7 concentration. The reactive radical species were determined indirectly by way of radical quenching tests, as well as directly by in situ electron paramagnetic resonance technique spin trapping tests. In a 15 min reaction using 5 mM PMS, a CuO dosage of 0.1 g/L, at pH 6.65, 95.38% of 40 mg/L AO7 can be removed. This study provides an effective oxidative system for AO7 removal by CuO/PMS heterogeneous process at ambient temperature.
Sulfate radical-based (SO4•−) advanced oxidation processes (SR-AOPs) have received a great deal of attention in recent years for removing environmental organic contaminants, such as phenol, dyes, perfluorinated compounds and pharmaceuticals (Ahmad et al. 2013; Duan et al. 2015; Oh et al. 2016). SO4•− can be generated from peroxymonosulfate (PMS) and peroxodisulfate (PDS) by UV, heat, base or transition metals (Guan et al. 2013; Qi et al. 2015). Compared with PDS, PMS is easier to activate by transition metals due to the asymmetric molecular structure (Anipsitakis & Dionysiou 2004). Homogeneous catalytic oxidation by PMS coupled with transition metals showed efficient decontamination of organic pollutants. Among the tested transition metals including Ag(I), Ce(III), Co(II), Fe(II), Fe(III), Mn(II), Ni(II), Ru(III) and V(III), Co(II) is the most effective catalyst of PMS activation for the production of sulfate radicals (Ding et al. 2013; Hu & Long 2016). However, cobalt has been recognized as a possible human carcinogen by the International Agency for Research on Cancer (IARC). Thus, using cobalt as a catalyst has limitations in practical applications. Consequently, the development of efficient catalysts for PMS activation remains a priority.
Copper-based catalysts have been intensively studied in recent years, and copper is not regarded as a potential carcinogen. In particular, CuO has been used with peroxymonosulfate (PMS) and peroxydisulfate (PDS) for the degradation of organic contaminants (Liang et al. 2013; Zhang et al. 2014; Du et al. 2017; Hu et al. 2017; Li et al. 2017b; Luo et al. 2018). The preparation methods for CuO in these studies used Cu(NO3)2.3H2O calcined in a closed muffle under high temperature (300–450 °C) or prepared by a hydrothermal method under high temperature (180–200 °C) and high pressure. However, high calcination temperature and the complicated preparation methods may hamper wider application. The size of the hydrothermal reactors (Teflon-lined autoclave) and the requirement of high pressure significantly limit preparation on a large scale. So, it is of great interest to study an economical and efficient method of preparing CuO and its use in the activation of persulfates for organic pollutant degradation.
Azo dyes have the -N = N- chromophoric group and may be monoazo, diazo or polyazo dyes, and are widely used by the textile industries, accounting for over 50% of all commercial dyes (López-López et al. 2007; Li et al. 2017a). These dyes are difficult to degrade by biological treatment methods due to their complex structure and their stability (Brillas & Martínez-Huitle 2015; Li et al. 2015). Therefore, in this study, we report a facile and low-cost method for the preparation of CuO. Acid Orange 7 (AO7), a typical azo dye and commonly used as a model compound for the study of dye degradation, and the effects of various reaction parameters such as initial PMS and AO7 concentrations, CuO dosage and solution pH are investigated. Furthermore, the electron paramagnetic resonance technique (EPR) and radical scavenging experiments using ethanol (EtOH), tert-butanol (TBA) and AO7 have been performed to investigate the radicals generated in the reaction system.
Chemicals and materials
PMS (2KHSO5·KHSO4·K2SO4 as Oxone) and 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO), were purchased from Sigma-Aldrich (St. Louis, MO, USA). Cupric nitrate (Cu(NO3)2·3H2O), ammonia (NH3.H2O), sodium hydroxide (NaOH), sulfuric acid (H2SO4), ethanol (EtOH), tert-butyl alcohol (TBA) and other chemical agents were purchased from Sinopharm Chemical Reagent Co., Ltd, (Shanghai, China). All chemicals were at least analytical grade and were used as received without further purification. A basic FE28 pH meter (Mettler Toledo, Shanghai, China) was used to measure the pH values. Ultrapure water (18.2 MΩ·cm) from a Milli-Q academic water purification system was used to prepare all the solutions required in the present work.
Preparation of CuO and characterization
CuO catalyst was fabricated using a modified simple and one-step hydrothermal method according to the previously reported method (Lei et al. 2015). Typically, 4.8 g Cu(NO3)2·3H2O was dissolved in ultrapure water (40 mL) to obtain a mixture solution and transferred into a 50 mL glass conical flask, followed by the addition of NH3.H2O. The mixture was stirred vigorously and heated in a water bath at 80 °C for 6 h, and aerated throughout the heating process. After being cooled to room temperature, the mixture was then centrifuged, and the desired solids were collected and dried at 105 °C. The surface morphology was characterized on an SU-8010 microscope (Hitachi, Japan). The crystal structure was characterized by X-ray diffraction (XRD, Rigaku S2, Japan).
Catalytic degradation experiment
The degradation experiments were carried out in a 250 mL conical flask at 25 °C. Desired amount of PMS and AO7 stock solutions were added to solution to a final volume of 200 mL. Solution pH was adjusted by H2SO4 or NaOH. Then the reaction was initiated once the catalyst was added. During reaction processes, the solution was stirred with a magnetic stirrer to ensure a complete mixing state. At the given reaction time intervals, approximately 1 mL of sample was withdrawn and the AO7 content was determined immediately (within 30 s).
RESULTS AND DISCUSSION
Figure 1 shows the representative scanning electron microscopy (SEM) images of CuO particles. The SEM observation indicated that CuO was composed of fine particles with grain sizes of 2–5 μm. Each microsphere is composed of small nanoleaflets with one end growing together to form a center and another end radiating laterally from this center. The XRD pattern is shown in Figure 2. The peaks of the sample match well with JCPDS card no. 01-089-2530 (Rao et al. 2018). All the diffraction peaks could be readily indexed to the monoclinic CuO. The result also showed a highly crystalline and single-phase structure of CuO.
Catalytic oxidation of AO7
The reactivity of CuO as PMS catalyst was studied by the reaction of degradation of AO7. As a comparison, experiments were carried out in different systems (Figure 3). It can be clearly seen that only 2.69% and 3.38% of AO7 was decolorized within 15 min reaction time in the control systems with only either PMS and Cu2+, respectively (with a total Cu amount the same as that in the used CuO). On the contrary, nearly 95% of AO7 was decolorized within 15 min in the CuO/PMS system. Obviously, CuO as a catalyst significantly enhanced the decolorization efficiency of the AO7.
Effect of catalyst dosage
The influence of catalyst amount on the AO7 degradation was studied by varying the concentration of catalysts from 0.05 to 0.20 g/L. At the given dosage of PMS (5 mM), the decolorization efficiency increased remarkably with increasing of the catalyst dosage (Figure 4), and the values of kobs increased from 0.00398 to 0.5210 min−1 when CuO concentration was raised from 0.05 to 0.2 g/L. This was because the increasing amount of catalyst could increase the number of active sites, which is beneficial to the formation of more reactive radicals. For example, at 0.05 g/L CuO, only 21.89% AO7 decolorization efficiency occurred in 5 min; however, when the concentration of CuO was 0.2 g/L, the decolorization efficiency could reach 96.12%, an enhancement of AO7 removal rate of over four times.
Effect of concentration of PMS
Effect of initial AO7 concentration
The effect of the initial concentration of AO7 was investigated under different initial concentrations of AO7 (20, 30, 40 and 50 mg/L) and the results are shown in Figure 6. It can be observed that AO7 decolorization efficiency decreased with increasing initial AO7 concentrations. The rate constant values (kobs) for AO7 decolorization at initial AO7 concentrations of 20–50 mg/L were from 0.3088 to 0.1766 min−1. For the whole range of initial concentrations, AO7 decolorization efficiencies of 90.39–95.38.0% were achieved within 15 min, which indicates that the CuO/PMS system has excellent AO7 decolorization performance at various concentration levels. When initial AO7 concentration is low, radical production will be higher than the consumption rate leading to a higher decolorization efficiency, while more reaction time for complete decolorization of AO7 is required at higher concentrations of AO7.
Effect of pH
The decolorization of AO7 was investigated at different initial pH values and the results are presented in Figure 7. Results revealed that increasing pH led to an increase in decolorization efficiency. PMS and AO7 had pKa values of 9.4 and 11.4, respectively (Ball & Edwards 1956; Bandara et al. 1999). PMS mainly existed as HSO5− in the solution at acidic and neutral pH values, and a small fraction of PMS existed as SO52− at pH below 9.4 (Zhang et al. 2013). Meanwhile, PMS existed mostly in the form of SO52− at pH range of 9.5–10.5, and SO52− was easier to activate than HSO5− in heterogeneous catalytic oxidation (Guan et al. 2013). AO7 mainly existed in the form of mono-anion (AO7−) and further dissociated into the dianion form (AO72−) at pH > 11.4. Overall, in this system, it is more favorable for the decolorization of AO7 under base condition (pH > 9). This result was consistent with previous reports that AO7 can be efficiently decolorized in a base/PMS system (Deng et al. 2017).
Effect of HCO3−
It is well known that bicarbonate can not only serve as a radical scavenger for •OH and SO4•– (Equations (4) and (5)) (Deng et al. 2013a, 2013b), but can also strongly influence solution pH. It was observed that decolorization rate increased with increasing HCO3− concentration from 0 to 10 mM (Figure 8), a phenomenon which may be attributed to three reasons. First, PMS can also be activated at high HCO3− concentrations. Yang et al. (2010) reported that AO7 can be efficiently decolorized in HCO3−/PMS system and inferred that the generation of SO4•– played the decisive role. Second, due to the buffering ability of NaHCO3, pH remained in the 8.11–9.96 range during the reaction process, AO7 can be efficiently decolorized in base/PMS system, and singlet oxygen (1O2) and superoxide anion radical (O2•—) were the dominant reactive oxygen species in AO7 decolorization (Deng et al. 2017). Third, carbonate radical (CO3•—) is a selective one-electron oxidant (E0 = 1.78 V at pH 7) and has varied reactivity towards organic compounds (Neta et al. 1988; Liu et al. 2016).
Reactive species and possible mechanism
To identify the radical species generated in the CuO/PMS process, radical quenching tests were performed using TBA and EtOH, respectively. Hydroxyl radical reaction can be scavenged by TBA (kTBA, •OH = (3.8–7.6) × 108 M−1 s−1), while the sulfate radical reacts with TBA much more slowly (kTBA, SO4•— = (4.0–9.1) × 105 M−1 s−1) (Neta et al. 1988). EtOH can be used to capture •OH and SO4•—(kEtOH, •OH = (1.8–2.8) × 109 M−1 s−1; kEtOH, SO4•— = (1.6–7.7) × 107 M−1 s−) (Jiang et al. 2015). As depicted in Figure 9(a), the presence of 1 M TBA significantly suppressed the oxidation process, with the decolorization efficiency decreasing from 95.38% to 68.61%, indicating that •OH were produced in the CuO/PMS system. For the EtOH-added test, the presence of 1 M EtOH significantly suppressed the oxidation process, with the decolorization efficiency decreasing from 95.38% to 32.52%. One possible reason could be if a greater amount of •OH existed in the system accompanied with negligible SO4•—, EtOH reacts faster with •OH than TBA, thus leading to more •OH being scavenged in the EtOH-added system than the TBA-added system. Consequently, a stronger inhibition effect on the decolorization efficiency could be observed in the EtOH-added system.
To further distinguish the reactive radical species in the CuO/PMS system, an EPR test with DMPO was employed to detect the radicals produced. As can be seen from Figure 9(b), the characteristic peaks of DMPO-OH and DMPO-SO4 adducts did not appear, but as the reaction proceeded, EPR signals with an intensity ratio of 1:2:1:2:1:2:1 was observed, which can be assigned to the characteristic signals of 5,5-dimethylpyrroline-(2)-oxyl-(1) (DMPOX), the nitroxide radical of DMPO. The presence of DMPOX signals can be related to the fast activation of PMS and efficient oxidation of DMPO (Li et al. 2017a), indicating highly effective activation of PMS in the CuO/PMS system.
TOC removal, specific oxidant efficiency and degradation pathways of AO7
TOC of AO7 solution (200 mL 40 mg/L AO7, 0.1 g/L CuO, 5 mM PMS and pH 6.65) was measured to analyze the mineralization. However, the TOC removal of AO7 was only 10.9% in the 15 min reactions, and the SOE value was 3.6. That is to say, the CuO/PMS system just led to the decomposition of AO7 molecules, while it cannot provide enough oxidation ability for the mineralization of residual organic molecules, especially with a low PMS dosage; low SOE was related to low TOC removal. The intermediates of AO7 degradation were detected by LCMS-IT-TOF (Figure 10). According to these detected intermediates and references (Lin et al. 2014; Shang et al. 2017), possible degradation pathways of AO7 are proposed in Figure 11.
In this study, low cost, safe and environmentally friendly CuO was synthesized through a facile one-step hydrothermal method, and then used as a PMS activator for the degradation of AO7. Radical quenchers including EtOH and TBA were used to investigate aspects of the mechanism and indicated that •OH and SO4•— were both involved in the decolorization of the AO7. Both •OH and SO4•— were suggested as the radical species in the CuO/PMS system. It was found that CuO had a high catalytic activity to degrade AO7 in the presence of PMS. Experimental results showed that AO7 decolorization efficiency was dramatically influenced by operating parameters, such as PMS concentration, CuO dosage and initial AO7 concentration. After 15 min reaction time using 5 mM PMS, a CuO dosage of 0.1 g/L, and initial pH value of 6.65, 95.38% of 40 mg/L AO7 was removed. This finding has great potential for the oxidative treatment of industrial waste and contaminated water. All the results indicated that the CuO/PMS catalytic oxidation was an efficient and clean technology for toxic and refractory industrial wastewater. Therefore, the green oxidation system consisting of CuO/PMS has great potential applications in the degradation of highly stable toxic organic pollutants.
We thank the China Postdoctoral Science Foundation (2017M620045) and Technology Innovation of Chongqing (No. cstc2015shmszx20012).
CONFLICT OF INTEREST
The authors state no conflict of interest.