Advanced oxidation processes play an important role in the removal of organic pollutants from wastewater, in which it is essential to develop an eco-friendly, effective, stable, and inexpensive catalyst. Herein, waste eggshell-supported copper oxide (CuO/eggshell) was synthesized via a facile method and employed as peroxymonosulfate (PMS) activator for the elimination of reactive blue 19 (RB19). CuO/eggshell exhibited high degradation efficiency of RB19 (approximately 100%) by activation of PMS under the optimum conditions of 20 mg/L RB19, 0.2 g/L CuO/eggshell, 0.36 mM PMS, and initial pH 7.12 within 20 min. In addition, the effects of catalyst dosage, PMS concentration, initial pH, inorganic ions, and humic acid on RB19 degradation were investigated. Scavenging experiments and electron paramagnetic resonance revealed that multiple reactive oxygen species, including sulfate radicals (SO4·−), hydroxyl radicals (·OH), superoxide radicals (O2·−), and singlet oxygen (1O2), contributed to RB19 degradation, and 1O2 played a dominant role. Finally, a possible PMS activation mechanism was proposed. This study suggests that loading catalytically active components onto waste eggshell is eco-friendly and effective for enhancing the degradation of dyes from wastewater.

  • CuO/eggshell was synthesized through a simple impregnation method.

  • CuO/eggshell exhibited excellent performance for PMS activation.

  • CuO/eggshell and PMS system can effectively degrade RB19 in.a wide pH range (2–12).

  • Singlet oxygen dominated the degradation of RB19.

Graphical Abstract

Graphical Abstract
Graphical Abstract

The discharge of recalcitrant organic compounds (ROCs), such as antibiotics, pesticides, and antidepressants, makes water pollution an increasingly challenging environmental problem. Dyes, one of the largest groups of organic compounds, are extensively used in textiles, paper, leather, and other industries (Qian et al. 2020). It is estimated that 10–15% of total dyes are used invalidly, resulting in more than 50,000 tons of dyes being discharged into the water environment per year (Le et al. 2011; Zhou et al. 2021). Commercial dyes are highly characterized by low biodegradability, carcinogenicity, teratogenicity, and mutagenicity (Yan et al. 2020). Moreover, the degradation products of some dyes show greater toxicity than their parent compounds (Oh et al. 2015). Traditional technologies, including coagulation/flocculation (Beluci et al. 2019), adsorption (Huang et al. 2021), and biological treatment (Cai et al. 2021), have been widely used to remove dyes from wastewater. The widespread application of these methods is mainly limited to the incomplete degradation of contaminants, the generation of sludge, or the transmission of pollutants from one phase to another (Nasiri et al. 2021). Therefore, it is essential to explore advanced treatment techniques.

Advanced oxidation processes based on persulfate (both peroxydisulfate and peroxymonosulfate) have been reported to be one of the most effective methods for removing ROCs benefiting from the strong oxidation ability, fast reaction rate, and high adaptability (Wang & Wang 2018). Reactive oxygen species (ROS) such as sulfate radicals (SO4·−), hydroxyl radicals (·OH), superoxide radicals (O2·−), and singlet oxygen (1O2) can be generated during the activation of persulfate (Li et al. 2020c; Zhu et al. 2021). Generally, persulfate can be activated by many methods, including thermal (Hori et al. 2008), ultraviolet light (Bao et al. 2018), ultrasound (Wei et al. 2017), and carbon materials (Luo et al. 2021). In recent decades, the activation of persulfate by transition metal oxides has received increasing attention due to their high activity, low energy consumption, and operational simplicity (Guo et al. 2021; Zhao et al. 2022). Among various transition metal oxides, CuO nanoparticles has gained increasingly attractive owing to its high efficiency, low cost, and low toxicity (Ji et al. 2014). In order to reduce metal leaching and prevent nanoparticles from aggregating into larger ones, the development of synthetic methods is still essential for practical application. The utilization of rigid support for CuO nanoparticles is a simple and effective strategy.

Many carriers have been developed, such as Al2O3 (Gawande et al. 2016), hexagonal boron nitride (Yan et al. 2019), biochar (Li et al. 2020c), and reduced graphene oxide (Du et al. 2019). Compared to loading CuO onto these high-cost supports, loading CuO onto available solid waste is environmentally sustainable and economical (Wang et al. 2020b). For example, Au/CuO/oyster shell possessed higher catalytic efficiency for CO oxidation (Liu et al. 2019). Among various solid waste, waste bird eggshells (such as chicken, duck, ostrich, and quail) are easily available natural biomaterials. It was suggested that about 8 million tons/year of waste eggshells are produced worldwide (Lin et al. 2021). The eggshell consists of approximately 94% CaCO3, 1% Ca3(PO4)2, 1% MgCO3, and 4% organic materials (mainly proteins) (Nasrollahzadeh et al. 2016). CaCO3, which contains functional groups such as C = O and C − O, has a high ability to capture and bond metal ions (Zhang et al. 2019). The eggshell structure can be divided into three layers: a thin mammillary inner layer without pores, a thick palisade middle layer containing many large pores, and a thin cuticle outer layer with no pores (Tan et al. 2015). There are approximately 7,000–17,000 funnel shaped canals distributed unevenly on the eggshell surface, which makes it very conducive to energy and mass transfer (Guo et al. 2019). Therefore, waste eggshell, an excellent bio-template, can be recycled as an economical and eco-friendly carrier of metal nanomaterials. It is meaningful to build a bridge between CuO nanoparticles and waste eggshell. On one hand, metal leaching and nanoparticles aggregation can be relieved. One the other hand, converting waste eggshell into resources has cost-saving economic benefits. However, the related composite materials for PMS activation were not investigated previously.

In this study, CuO/eggshell catalysts were prepared in a simple way and used as activators of peroxymonosulfate (PMS). Reactive blue 19 (RB19) was chosen as the target dye to evaluate the catalytic performance of CuO/eggshell. Several key parameters, including the catalyst dosage, PMS concentration, and initial pH, were investigated. The stability and reusability of CuO/eggshell was investigated to confirm their potential for practical applications. The effects of inorganic ions (chloride (Cl), nitrate (NO3), and carbonate (CO32−)) and natural organic matter (humic acid, HA) on the removal of RB19 were also evaluated. In addition, quenching experiments and electron paramagnetic resonance analysis were conducted to determine the ROS. Finally, a possible mechanism for CuO/eggshell in PMS activation was proposed.

Materials

Chicken eggshells were collected from household kitchen waste. Potassium peroxymonosulfate (PMS; KHSO5·0.5KHSO4·0.5 K2SO4) was obtained from Alfa Aesar (Germany; purity >98.5%). RB19 was obtained from Aladdin Industrial Corporation (China; purity >98.5%). All other chemicals were of analytical grade and were purchased from Aladdin Industrial Corporation (China). Deionized water was used in this study.

Preparation of CuO/eggshell

The eggshells were carefully rinsed several times with deionized water and dried at 80 °C to a constant weight. The dry eggshells were ground into powder and sieved through 100-mesh. The eggshell powder was then treated with 0.01 M HCl and stirred for 1 h to remove surface impurities. Finally, the treated eggshell powder was thoroughly washed with deionized water and dried at 80 °C.

CuO/eggshell composites were synthesized using a facile impregnation method. Briefly, 1.2481 g (0.005 mol), 2.4962 g (0.01 mol), and 4.9924 g (0.02 mol) of CuSO4·5H2O were dissolved in 100 mL of deionized water, respectively. The mixture was stirred under magnetic stirring at room temperature for 16 h after adding 1 g of HCl-treated eggshell powder into the above CuSO4·5H2O solutions. 1 g eggshell is about 0.01 mol CaCO3 for its main component is CaCO3 (Gao et al. 2021). The resulting blue solids were collected, washed several times with ethanol and deionized water, and dried at 80 °C. Wei et al. (2009) indicated that CaCO3 decomposed into CaO above 600 °C. Therefore, the dried product was powdered and further calcined in a muffle furnace at 600 °C for 2 h. Finally, these different CuO loaded-eggshell catalysts were defined as 0.5CuO/eggshell, CuO/eggshell, and 2CuO/eggshell. For comparison, pure CuO nanoparticles were prepared by calcination of Cu (OH)2 from CuSO4·5H2O and NaOH at 600 °C for 2 h.

Characterization of CuO/eggshell

The morphologies were determined using a field emission scanning electron microscopy (FESEM, ZEISS, Germany) coupled with an energy dispersive X-ray (EDX) and transmission electron microscopy (TEM, Tecnai G2 F20, USA). The crystalline structures of the prepared samples were characterized by X-ray diffractometer (XRD, D8 Advance, Bruker, Germany) operated at 40 kV and 40 mA with a Cu Kα radiation source (λ = 0.15406 nm). Fourier transform infrared (FTIR) spectra were measured on a Thermo Nicolet spectrophotometer (iS10, USA). The chemical compositions were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI, USA).

Catalytic degradation experiments

All experiments were performed in 250 mL glass beakers containing 100 mL of 20 mg/L RB19 at room temperature (25 °C) under continuous stirring at 300 rpm. NaOH (0.1 M) or HCl (0.1 M) was used to adjust the pH of the original solution. During each operation, a required number of catalysts was added into the solution, followed by 10 min of stirring to uniformly disperse the catalysts. Subsequently, quantitative PMS was added to initiate the catalytic process. At a specific time, a volume of samples was collected and filtered through a 0.45 μm membrane, followed by immediate analysis with a UV-vis spectrometer (Shimadzu, Japan) at λmax 592 nm (Ileri & Dogu 2022). The effects of CuO loading in CuO/eggshell, catalyst dosage, PMS concentration, and initial pH on RB19 removal were investigated. In addition, various concentrations of Cl, NO3, CO32−, and HA were added into the system to study their influences on RB19 degradation. To identify the possible ROS generated from CuO/eggshell and PMS system, the quenching experiments and electron paramagnetic resonance (EPR) with 100 mM 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and 2,2,6,6-tetramethyle-4-piperidone (TEMP) as spin-trapping agents on JES FA200 EPR (JEOL, Japan) were carried out. The concentration of leached Cu ions after the reaction was determined using inductively coupled plasma optical emission spectrometer (ICP-OES, PerkinElmer Optima 7000DV, USA). All experiments were performed three times, and all data are indicated as means with error bars.

Characterization of catalyst

The XRD patterns of the samples were shown in Figure 1. For CuO/eggshell composites, the main peaks at 2θ at 32.49°, 35.50°, 38.77°, 48.66°, 53.43°, 58.37°, and 61.49° corresponded to the (1 1 0), (0 0 2), (1 1 1), (−2 0 2), (0 2 0), (2 0 2), and (−1 1 3) planes of monoclinic CuO, respectively, which matched well with PDF no. 80-1917. The corresponding 2θ at 23.02°, 29.40°, and 31.41° could be assigned to the (0 1 2), (1 0 4), and (0 0 6) of hexagonal CaCO3 indexed to PDF no. 05-0586. The diffraction peaks of CaCO3 were largely weakened, which may be due to the generation of impurities. The diffraction peaks at 25.43°, 28.57°, 31.36°, 41.31°, 43.33°, and 55.72° can be ascribed to the (0 2 0), (0 0 2), (0 1 2), (3 0 1), (1 3 1) and (2 3 2) planes, which were assigned to orthorhombic CaSO4 (PDF no. 37-1496). However, the XRD pattern of pure eggshell was consistent with that of CaCO3. CaSO4 may be produced by the adsorption of SO42− onto eggshell. This result was consistent with EDX elemental mapping analysis discussed below (Figure 2(d)).

Figure 1

X-ray diffraction patterns of as-prepared samples.

Figure 1

X-ray diffraction patterns of as-prepared samples.

Close modal
Figure 2

FESEM (a), TEM (b), high-resolution TEM (c), and EDX images of CuO/eggshell and corresponding element mapping images of Cu, O, Ca, C, and S (d).

Figure 2

FESEM (a), TEM (b), high-resolution TEM (c), and EDX images of CuO/eggshell and corresponding element mapping images of Cu, O, Ca, C, and S (d).

Close modal

The surface property of CuO/eggshell was further characterized by FT-IR (Fig. S1). The absorption bands at 3,570.18 and 3,485.55 cm−1 could be assigned to the surface-bonded H2O molecule (Zhang et al. 2019). Two weak bands at 2,925.12 and 2,854.73 cm−1 indicated C-H stretching vibration (Gupta & Rathod 2018). The band centered around 1,140 cm−1 divided into two peaks at 1,151.83 and 1,118.84 cm−1 were ascribed to the stretching and bending vibration of sulfate (Böke et al. 2004). Bands at 881.35 and 712.30 cm−1, whose peak strengths were significantly decreased compared to those of pure eggshell, was resulted from the in-plane deformation and out-of-plane deformation vibration of the carbonate anions (Tofail et al. 2019; Zhang et al. 2019). The bands at 676.92, 612.31, 594.96 and 489.36 cm−1 could be assigned to Cu-O vibration, which suggested the existence of CuO (Du et al. 2019; Zhang et al. 2019).

The morphologies and structures of the samples were investigated by FESEM and TEM. Fig. S2 showed that the pure eggshell exhibited a rough surface containing many irregular and rigid pore structures, and the pure CuO nanoparticles had a rod-like structure with an uneven surface. Compared to pure CuO and eggshell, the morphology of CuO/eggshell underwent significant changes. As shown in Figure 2(a), the surface of raw eggshell was fully covered with particles. To gain more insight into the morphology of CuO/eggshell, TEM and high-resolution TEM (HR-TEM) were carried out with the results displayed in Figure 2(b) and 2(c). As can be observed, CuO nanoparticles were attached to the surface of eggshell. As shown in Figure 2(c), the lattice fringes with a spacing of 0.2757 nm and 0.3508 nm could be assigned to the (1 1 0) plane of CuO and (0 2 0) plane of CaSO4, respectively. However, the interplanar spacing attributed to CaCO3 was not observed in the HR-TEM images, which may be due to the fact that small quantities of CaCO3 were covered with CaSO4 in large quantities. To further verify these results, EDX was used to measure the elemental composition and distribution. As shown in Figure 2(d), a relatively uniform distribution of Cu, O, Ca, C, and S was observed, indicating that S was introduced into the CuO/eggshell.

XPS was used to determine the surface composites and oxidation states of the CuO/eggshell. The survey spectrum of CuO/eggshell demonstrated the presence of Cu, O, Ca, C, and S elements (Figure 3(a)). The Cu 2p XPS spectrum displayed two characteristic peaks of the Cu 2p3/2 and Cu 2p1/2 centered at 934.04 and 954.03 eV with a splitting width of 19.9 eV (Figure 3(b)) (Park et al. 2009). The other two obvious satellite peaks at 962.04 and 943.76 eV, which were characteristic peak signals of Cu2+, further confirming the presence of CuO (Oh et al. 2020). The O 1 s XPS spectrum exhibited two major peaks at 529.42 and 531.43 eV, which were assigned to lattice oxygen and surface hydroxyl groups, respectively (Figure 3(c)) (Ma et al. 2019a). As revealed in Figure 3(d), the main peaks at 347.27 and 350.77 eV can be denoted to Ca 2p3/2 and Ca 2p1/2, respectively, suggesting the presence of oxidized Ca2+ in Ca species (Wang et al. 2006). As shown in Figure 3(e), three peaks fitted at 284.77, 286.37, and 288.52 eV in C 1 s spectrum could be attributed to C-C, C-O, and O = C-O, respectively (Yu et al. 2017). Regarding S 2p, there was a significant peak centered at 168.76 eV (Figure 3(f)), which could be ascribed to SO42− species (Bai et al. 2020).

Figure 3

XPS survey (a), Cu 2p (b), O 1 s (c), Ca 2p (d), C 1 s (e), and S 2p (f) XPS spectra of CuO/eggshell.

Figure 3

XPS survey (a), Cu 2p (b), O 1 s (c), Ca 2p (d), C 1 s (e), and S 2p (f) XPS spectra of CuO/eggshell.

Close modal

Catalytic performance of CuO/eggshell

Degradation efficiency in different processes

As exhibited in Figure 4(a), PMS alone or CuO/eggshell alone showed a marginal removal of RB19, suggesting that the contribution of PMS self-oxidation and CuO/eggshell self-adsorption could be neglected. In addition, the eggshell and PMS system also showed an insignificant RB19 removal, indicating that the eggshell could not act as a catalyst to promote the activation of PMS. However, a significant improvement was observed in the CuO/eggshell and PMS system, which could remove 99.72% of RB19 within 20 min. Although the CuO and PMS system could degrade 99.05% of RB19 within 20 min, it was not suitable as catalysts because of the easy aggregation of CuO nanoparticles. Moreover, for the same catalyst dosage, the amount of Cu used by CuO was much higher than that of CuO/eggshell. The eggshell was beneficial for preventing aggregation of CuO nanoparticles and promoting the dispersion of CuO nanoparticles. The above results indicated that the synergic effect between CuO and eggshell can result in high catalytic performance. The effect of CuO loading on catalytic performance was also investigated. As can be seen in Fig. S3, the higher CuO loading resulted in higher RB19 degradation. However, the degradation rate decreased slightly with further increase of CuO loading. Excessive loading can lead to aggregation of catalyst particles and reduction of active sites, thus inhibiting the catalyst activity (Pang et al. 2020).

Figure 4

The degradation efficiency of RB19 in different systems (a). Experimental conditions: [RB19] = 20 mg/L, [catalysts] = 0.1 g/L, [PMS] = 0.36 mM, initial pH = 7.12 (unadjusted), stirring speed = 300 rpm, T = 25 °C. Effects of catalyst dosage (b), PMS concentration (c), and initial pH (d) on RB19 degradation. Experimental conditions: [RB19] = 20 mg/L, [CuO/eggshell] = 0.2 g/L, [PMS] = 0.36 mM, initial pH = 7.12 (unadjusted), stirring speed = 300 rpm, T = 25 °C.

Figure 4

The degradation efficiency of RB19 in different systems (a). Experimental conditions: [RB19] = 20 mg/L, [catalysts] = 0.1 g/L, [PMS] = 0.36 mM, initial pH = 7.12 (unadjusted), stirring speed = 300 rpm, T = 25 °C. Effects of catalyst dosage (b), PMS concentration (c), and initial pH (d) on RB19 degradation. Experimental conditions: [RB19] = 20 mg/L, [CuO/eggshell] = 0.2 g/L, [PMS] = 0.36 mM, initial pH = 7.12 (unadjusted), stirring speed = 300 rpm, T = 25 °C.

Close modal

Effect of catalyst dosage

The effect of catalyst dosage on RB19 degradation in the CuO/eggshell and PMS system was illustrated in Figure 4(b). The degradation efficiency of RB19 increased from 26.48% to 99.72% when the catalyst dosage increased from 0.025 to 0.1 g/L. With the catalyst dosage continued to increase to 0.2 g/L, the RB19 degradation was greatly improved, and approximately 100% of RB19 was removed within 10 min. Increasing catalyst dosage provided more active sites to activate PMS and generate ROS to achieve high RB19 degradation. Hence, we chose 0.2 g/L of catalyst dosage to continue the subsequent experiments.

Effect of PMS concentration

As shown in Figure 4(c), a significantly increased degradation efficiency of RB19 from 23.70% to 87.75% at 8 min was observed when the PMS concentration increased from 0.18 mM to 0.36 mM, which might be attributed to the generation of more ROS. When the PMS concentration increased from 0.36 mM to 1.44 mM, the degradation rate of RB19 still improved, but the degree of improvement was lower than that from 0.18 mM to 0.36 mM. This phenomenon may be due to the limited number of active sites on the surface of catalyst, resulting in a low ability to react with a large amount of PMS. Considering the PMS cost and the RB19 removal efficiency, 0.36 mM PMS was used for further experiments.

Effect of pH

The solution pH played an important role in RB19 degradation in the CuO/eggshell and PMS system, as illustrated in Figure 4(d). Acidic condition was unfavorable for the degradation process, and the removal efficiency was only 15.49% at pH = 2. When the pH was adjusted to 4 and 6, the degradation efficiency increased to approximately 50% and 60% within 20 min, respectively. The RB19 removal increased dramatically, reaching 95% at 10 min, when the initial pH maintained unadjusted (pH = 7.12). The degradation efficiency increased rapidly within 2 min when the pH increased to 10. There was no significant improvement on RB19 removal when the pH increased from 10 to 12. The above results suggested that the obtained catalyst possessed high pH resistance.

Under acidic conditions, Cu was highly leached, resulting in the inhibition of PMS activation. This can be observed in Fig. S4, the concentration of copper ions reached 76.48 ± 3.72 mg/L at pH = 2 after 20 min reaction, while there was almost no Cu leached under alkaline conditions. In addition, it was suggested that ·OH and SO4·− could be scavenged quickly in the presence of many H+ ions in solution (Gao et al. 2021).

Under alkaline conditions (pH ≥ 10), many studies have reported that the degradation of organic pollutants was greatly inhibited (Ma et al. 2019a; Guo et al. 2020; Gao et al. 2021). There were several reasons for this phenomenon: (i) excessive OH could react with SO4·− to produce less active ·OH (Li et al. 2016); (ii) ·OH and SO4·− have lower lifetimes in alkaline solutions (Ma et al. 2019b); (iii) the self-decomposition of PMS through a non-radical pathway at high pH (Rastogi et al. 2009). However, this study showed that the removal of RB19 was not inhibited at high pH. Without the addition of CuO/eggshell, the removal of RB19 by PMS alone was negligible at pH 12 (Fig. S5). This phenomenon may be due to the synergistic effect of radical and non-radical oxidation pathways.

Application potential

Some previous reports about RB19 degradation through different treatment processes were summarized on Table S1, which suggested that the CuO/eggshell and PMS system was suitable for RB19 degradation. The practical feasibility of CuO/eggshell in natural water (pond water, lake water, and river water) was evaluated. The samples of pond water (pH = 7.72) and lake water (pH = 7.29) were taken from the campus, and river water (pH = 7.88) was collected from Jin River in Quanzhou. Fig. S6a showed rapid degradation efficiencies in all water matrixes, indicating that CuO/eggshell can serve as a promising candidate for PMS activation in real wastewater treatment.

Under the optimum conditions, the stability and reusability of CuO/eggshell were evaluated. Spent CuO/eggshell was collected by vacuum filtration at the end of reaction process. Then, they were washed with deionized water several times and re-dispersed in the fresh RB19 solution. As revealed in Fig. S6b, CuO/eggshell showed good stability in RB19 degradation. The removal of RB19 exhibited an insignificant decrease after the first degradation reaction, and approximately 76% of RB19 could be degraded within 20 min after three cycles. The leaching concentration of Cu was 1.28 mg/L (Fig. S3), leading to a reduction in catalytic activity.

Influence of inorganic ions and natural organic matter

The effects of common inorganic anions (Cl, NO3, CO32−) and natural organic matter (HA) on RB19 degradation over CuO/eggshell and PMS system were investigated. As shown in Fig. S7a, the degradation rate of RB19 decreased slightly at low concentration of Cl (1 mM), and the removal curves of RB19 at higher Cl concentrations (5 and 10 mM) nearly overlapped that of without Cl addition. A similar phenomenon was observed in the sludge-activated carbon-supported CoFe2O4 and PMS system (Yang et al. 2020). It was suggested that ·OH and SO4·− could be consumed by Cl to produce less reactive chlorine or hypochlorous radicals (such as Cl· and HOCl·) at low concentrations, while Cl shows an enhancement effect by generating reactive halogens when the concentration exceeds the critical level (Wang et al. 2011; Guo et al. 2020).

Fig. S7b suggested that NO3 had a dual effect on RB19 removal in the CuO/eggshell and PMS system. Low concentrations of NO3 (1 and 5 mM) slightly accelerated the removal processes, which was like the study with Ag2O-Ag/eggshell as a PMS activator (Gao et al. 2021). However, high concentrations of NO3 (10 mM) slightly inhibited the degradation process. It was reported that 20 mM NO3 had a negative effect on the degradation of rhodamine B (Guo et al. 2020). NO3 could consume radical species to generate nitrate, but the reaction rate between NO3 and SO4·− or ·OH was 5.5 × 105 M1s1 and below 5.5 × 105 M1s1, respectively, which has the lowest reaction rate with SO4·− and ·OH compared with other ions (Cao et al. 2019). Therefore, the slight promotion effect with a low concentration of NO3 may be due to the low reaction of NO3 with SO4·− and ·OH, while the slight inhibition effect may be due to the increased reaction rate of NO3 with SO4·− and ·OH under high concentrations of NO3.

As illustrated in Fig. S7c, CO32− accelerated the degradation process at a concentration of 1–10 mM, but the enhancement decreased with the increase of CO32− concentration. In the study by Bai et al. (2020) the removal processes were enhanced by the addition of 1–10 mM CO32− in the first 5 min, although the CO32− ultimately had a detrimental effect on fluoxetine degradation. CO32− can react with SO4·− and ·OH to produce carbonate radicals, which might act as promoters to stimulate PMS decomposition into SO4·− or ·OH with lower concentrations of CO32−(Guan et al. 2013).

HA is a macromolecular organic substance which exists widely in nature. As shown in Fig. S7d, HA had a detrimental effect on RB19 removal. The degradation efficiency of RB19 decreased from approximately 100% to approximately 40%, 25%, and 21% when the HA concentration increased from 0 mg/L to 5, 10, and 20 mg/L, respectively. According to the report (Chen et al. 2018), this negative effect could be ascribed to (i) HA scavenges radical species; (ii) HA could block the active sites of the catalyst due to the strong π-π stacking.

Possible mechanism of PMS activation over CuO/eggshell

Identification of ROS

To reveal the PMS activation mechanism, the ROS were investigated through species quenching experiments with methanol (MeOH), tert-butanol (TBA), and L-histidine (L-his) as scavengers. MeOH has a high reactivity with both ·OH and SO4·−, TBA can be used as an ·OH scavenger because the reaction rate of TBA with ·OH is much higher than that of SO4·−(Yang et al. 2017). L-his is a well-known 1O2 quencher (Ma et al. 2019c). As Figure 5(a) illustrated, both MeOH and TBA showed slight inhibitions on RB19 degradation. Conversely, the RB19 degradation efficiency decreased significantly from 99.89% to 23.94% after the addition of 10 mM L-his. Based on the above results, SO4·−, ·OH, and 1O2 participated in the reaction, but 1O2 was the main contributor.

Figure 5

Effects of scavengers on RB19 removal (a); EPR spectra in CuO/eggshell and PMS system with DMPO (b, c) and TEMP (d) as spin-trapping regents. Experimental conditions: [RB19] = 20 mg/L, [CuO/eggshell] = 0.2 g/L, [PMS] = 0.36 mM, initial pH = 7.12 (unadjusted), [DMPO] = 100 mM, [TEMP] = 100 mM, stirring speed = 300 rpm, T = 25 °C.

Figure 5

Effects of scavengers on RB19 removal (a); EPR spectra in CuO/eggshell and PMS system with DMPO (b, c) and TEMP (d) as spin-trapping regents. Experimental conditions: [RB19] = 20 mg/L, [CuO/eggshell] = 0.2 g/L, [PMS] = 0.36 mM, initial pH = 7.12 (unadjusted), [DMPO] = 100 mM, [TEMP] = 100 mM, stirring speed = 300 rpm, T = 25 °C.

Close modal

To further identify the ROS, EPR spectroscopy was carried out. As shown in Figure 5(b), four typical peak signals (1:2:2:1) and six weak peak signals were detected, which were attributed to DMPO-·OH and DMPO-SO4·−, respectively. Moreover, the EPR also unveiled the existence of O2·− based on the signal characteristic of DMPO-O2·− (Figure 5(c)). A 1:1:1 signal characteristic of TEMP-1O2 was observed in 2 min, and the signal intensity increased significantly in 10 min (Figure 5(d)), confirming that 1O2 was largely produced in this system. The above results indicated that SO4·−, ·OH, O2·−, and 1O2 contributed to RB19 removal, but 1O2 played a dominant role in the CuO/eggshell and PMS system.

Possible mechanism of PMS activation

To better understand the activation mechanism of PMS over CuO/eggshell, high-resolution XPS spectra of the spent CuO/eggshell was measured. As revealed in Fig. S8a, after the catalytic reaction, the Cu 2p peak slightly shifted to a lower binding energy value. Moreover, the Cu 2p3/2 peak split into two peaks at Cu(II) (934.45 eV) and Cu(I) (932.9 eV) (Li et al. 2019; Wang et al. 2020a). The relative contents of Cu(II) and Cu(I) on the used CuO/eggshell surface were 31.71% and 68.29%, respectively. The results showed that the conversion of Cu(II) and Cu(I) on the surface of CuO/eggshell during the catalytic reaction were involved in the activation of PMS. For O 1 s, the ratio of surface hydroxyl groups increased with a decrease in lattice oxygen (Fig. S8b), suggesting that strong hydroxylation occurred on the CuO/eggshell surface during the reaction process.

According to the above results, a possible mechanism of PMS activation over CuO/eggshell was proposed (Figure 6). First, ≡Cu(II) on the CuO/eggshell surface could directly contribute to the generation of SO5·− (Equation (1)), which further promoted the production of SO4·− and 1O2 (Equations (2) and (3)) (Wang et al. 2019; Li et al. 2020c). Then, the produced ≡Cu(I) catalyzed the decomposition of PMS to generate SO4·− and ·OH (Equations (4) and (5)) (Li et al. 2017). The generated SO4·− could partially react with H2O and OH to produce ·OH (Equations (6) and (7)) (Gao et al. 2021). In addition, the hydroxylated CuO/eggshell generated Cu(II)-OH active sites, and Cu(II)-OH activated PMS to produce 1O2 (Equations (8) and (9)) (Li et al. 2020a, 2020b; Wang et al. 2020a). Moreover, PMS can decompose to produce H2O2 and further generated HO2· (Equations (10) and (11)) (Qiu et al. 2019; Gao et al. 2021). The formed HO2· could contribute to the generation of O2·− and 1O2 (Equations (12)–(15)). Finally, SO4·−, ·OH, O2·−, and 1O2 were all beneficial for the degradation of RB19, but 1O2 played a dominant role.
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Figure 6

Proposed mechanism for catalytic RB19 degradation over the CuO/eggshell and PMS system.

Figure 6

Proposed mechanism for catalytic RB19 degradation over the CuO/eggshell and PMS system.

Close modal

Based on all the results about the degradation of RB19 through the CuO/eggshell & PMS system, the conclusions were as follows:

  • (1)

    CuO/eggshell catalyst was synthesized through a wet-impregnation method. The CuO/eggshell exhibited a higher activity to activate PMS for RB19 degradation than CuO.

  • (2)

    Some key parameters, such as the catalyst dosage, PMS concentration, initial pH, inorganic ions, and humic acid were tested and the results indicated that CuO/eggshell had well adaptability under different conditions.

  • (3)

    The scavenging experiments and EPR suggested that SO4·−, ·OH, O2·−, and 1O2 were beneficial for RB19 degradation, but 1O2 played a dominant role.

  • (4)

    To improve the catalytic performance of CuO/eggshell, more studies are required: (i) prepare different CuO/eggshell catalysts with CuSO4, Cu(NO3)2 or CuCl2 as precursors, and then compare their catalytic activity; (ii) synthesize magnetic CuO/eggshell for more easy recycling; (iii) improve the synthesize method to promote the stability and adaptability to relieve the occurrence of metal leaching.

Overall, this work suggests that the waste eggshell can be recycled as a green support of transition metal oxides to promote the treatment of wastewater through PMS activation.

This work was financially supported by the National Natural Science Foundation of China (22006091) and Natural Science Foundation of Fujian Province (2021J05183) and Fujian Provincial Department of Educational Project (JAT190512 and JT180362). We thank Elsevier for providing editorial support and helping us improve the language of this paper.

The authors declare that they have no conflict of interest.

All relevant data are included in the paper or its Supplementary Information.

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