Constructing a brand-new advanced oxidation process system composed of MgO₂ nanoparticles and MgNCN/MgO nanocomposites for organic pollutant degradation

Abstract

A brand-new advanced oxidation process (AOP) system consisting of MgO₂ nanoparticles and MgNCN/MgO nanocomposites was firstly developed for the degradation of organic pollutants. In the novel AOP system, MgO₂ nanoparticles as oxidants and MgNCN/MgO nanocomposites as activators were both obtained through a simple one-step process. Batch experiments indicated that the MgO₂–MgNCN/MgO system could efficiently degrade organic pollutants in a wide pH range (4–12), and the degradation efficiency was hardly affected by common matrix species (Cl⁻, NO₃⁻, HCO₃⁻, SO₄²⁻ and humic acid) and water sources. The high stability and slow-release characteristic of MgO₂ endowed the AOP system with a long-term oxidation ability for continuous degradation of organic pollutants. The dominant reactive oxygen species (ROS) responsible for organic degradation was confirmed to be singlet oxygen (¹O₂), which was attributed to the activation of MgO₂ by the bonding effect between MgNCN and MgO in the nanocomposites. Finally, the generated ions, NH₄⁺, NO₂⁻ and NO₃⁻, and residual solid in the degradation solution were determined, and the process of organic degradation in the MgO₂–MgNCN/MgO system was proposed. This work not only constructed a brand-new AOP system for environmental remediation, but also offered a promising strategy for continuous treatment of organic-polluted wastewater and even soil.

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Environmental significance

Advanced oxidation processes (AOPs) are effective technologies for the degradation of organic contaminants. However, the reported AOP systems always suffer from some shortcomings or limitations such as short duration, toxic metal components, a narrow working pH range, and dependence on external energy excitation, which greatly restricted the practical application of these AOP systems in environmental remediation. In this work, a novel AOP system with high adaptability and long-term oxidation ability was firstly developed by using stable MgO₂ nanoparticles as oxidants and transition metal-free MgNCN/MgO nanocomposites as activators. The AOP system could efficiently degrade organic contaminants in a wide pH range (4–12), and was rarely affected by common matrix species (Cl⁻, NO₃⁻, HCO₃⁻, SO₄²⁻ and HA) and natural water sources. Singlet oxygen (¹O₂) was confirmed to be the main ROS responsible for organic degradation, which is generated from activating MgO₂ by the bonding effect between MgNCN and MgO in the nanocomposites.

1. Introduction

In the past decades, huge amounts of synthetic organic substances, such as dyes, insecticides and antibiotics, have been extensively used in industries and daily life. In addition to bringing benefits for humans, they have resulted in a serious problem of environmental pollution. The accumulation of these organic pollutants in the environment causes great harm to the ecological balance and human health, due to the fact that most of them are detrimental and difficult to be degraded naturally. Therefore, various approaches including adsorption,^1,2 biodegradation,³ pyrolysis,⁴ membrane filtration⁵ and advanced oxidation processes (AOPs)^6,7 have been widely investigated and applied for the effective removal of complex and bio-refractory organic pollutants. Among these techniques, AOPs have been regarded as one of the most attractive techniques, due to the generation of reactive oxygen species (ROS) with strong oxidizing ability (such as hydroxyl radicals (˙OH), sulfate radicals (SO₄˙⁻), superoxide radicals (O₂˙⁻) and singlet oxygen (¹O₂)), simple operation conditions and relatively high efficiency.^8–10

H₂O₂ is commonly used as an oxidant in AOPs to degrade organic pollutants, because it does not produce any other by-products except water and oxygen.¹¹ However, liquid H₂O₂ has poor stability and easily decomposes into water and oxygen in nature even at ambient temperature, which makes it difficult to be stored and transported. Therefore, different types of solid oxidants are developed to replace liquid H₂O₂ in environmental remediation. As potential alternatives to liquid H₂O₂, peroxydisulfate/peroxymonosulfate could be activated by transition metals, carbonaceous materials and their composites to produce radicals (˙OH/SO₄˙⁻)¹² and non-radicals (¹O₂)^13,14 for degrading organic pollutants. However, the process would release sulfate ions and metal ions into water, causing the problem of secondary pollution. Recently, metal peroxides such as CaO₂,¹⁵ MgO₂ (ref. 16) and ZnO₂ (ref. 17 and 18) have been reported for the Fenton/Fenton-like degradation of organic pollutants, owing to high stability, low cost and sustained release of H₂O₂ in water. Compared with other metal peroxides, MgO₂ possesses a higher active oxygen content,¹⁹ suggesting that a relatively low dosage of MgO₂ is required in practical application. Meanwhile, the hydrolysate of MgO₂ has a lower solubility product constant (K_sp (Mg(OH)₂) = 1.8 × 10⁻¹¹), which will cause a lower leakage of metal ions and a weaker impact on the system pH. More importantly, MgO₂ has a moderate release rate of H₂O₂ in aqueous solution,^20,21 contributing to the long-term oxidation for organic degradation. Thus, it can be expected that MgO₂ has greater potential in environmental remediation.

In a conventional AOP system, another key component is the catalyst or activator. Fe²⁺/Fe³⁺ are usually used to activate oxidants (H₂O₂, CaO₂, peroxymonosulfate) to generate ROS for organic degradation.^22–24 Unfortunately, the use of Fe²⁺/Fe³⁺ always results in some problems: requirements of acidification and alkali neutralization before and after degradation and generation of massive environment-harmful ferric sludge.^25–27 To overcome these shortcomings, various heterogeneous catalysts including metal compounds (Fe₃O₄, FeOOH, ZnNb₂O₆ and FeOCl),^28–31 metal composites (Au/CeO₂, Fe/Mn–SiO₂ and Co/N-doped graphene)^7,8,32 and metal-free catalysts (C₆₀/g-C₃N₄, graphene/g-C₃N₄ and PDA/g-C₃N₄)^33–35 have been investigated to replace homogeneous metal ions. However, most of the reported heterogeneous catalysts contain transition metal components such as Fe, Mn and Co, which can cause secondary pollution due to the leakage of toxic metal ions. Some heterogeneous catalysts can only work under the excitation of external energies (light, electricity, microwave and ultrasound), which means extra energy consumption and specific workplace constraints. On the other hand, although a few catalysts have been reported which could extend the working pH range to neutral or even weak alkaline,^36,37 almost no catalyst/activator was efficient in a strong alkaline (pH > 10) solution. These issues mentioned above greatly restrict the practical application of AOP systems in environmental remediation. Therefore, exploring environmentally friendly and broad-pH-applicable catalysts/activators without external energy activation is urgently required for the purpose of sustainable development.

Our previous work³⁸ demonstrated that the MgNCN/MgO nanocomposites could be used as H₂O₂ activators to degrade tetracycline in a wide pH range without external energy activation. Herein, we presented a brand-new AOP system using MgO₂ nanoparticles as oxidants and MgNCN/MgO nanocomposites as activators for the degradation of organic pollutants. Both MgO₂ nanoparticles and MgNCN/MgO nanocomposites were synthesized through a facile one-step process, and their morphologies and structures were confirmed by various characterization techniques. The degradation performances and adaptability of the MgO₂–MgNCN/MgO system towards organic pollutants were systemically investigated in terms of MgO₂ dosage, MgNCN/MgO dosage, initial solution pH, degradation temperature, common matrix species, different types of organic pollutants and water sources. Also, the slow-release characteristic and long-term degradation ability were explored by determining the concentration of released H₂O₂ and running the consecutive degradation experiment. The crucial role of reactive oxygen species (ROS) generated in the system was revealed by quenching tests and electron paramagnetic resonance (EPR) measurements. Additionally, the degradation process of organic pollutants in the MgO₂–MgNCN/MgO system was studied, based on the analyses of generated intermediates and residual solid in solution.

2. Experimental section

2.1. Materials

Magnesium nitrate (Mg(NO₃)₂·6H₂O), aqueous ammonia (NH₃·H₂O, 25–28 wt%), hydrogen peroxide (H₂O₂, 30 wt%), magnesium carbonate basic pentahydrate ((MgCO₃)₄·Mg(OH)₂·5H₂O), thiourea (CH₄N₂S), sulfuric acid (H₂SO₄), sodium hydroxide (NaOH), sodium chloride (NaCl), sodium nitrate (NaNO₃), sodium sulfate (Na₂SO₄), sodium bicarbonate (NaHCO₃), methylene blue (MB), orange G (OG), isopropanol (IPA) and furfuryl alcohol (FFA) were provided by Sinopharm Chemical Reagent Co., Ltd. Acid orange 7 (AO7) was obtained from Shanghai Macklin Biochemical Co., Ltd. Humic acid (HA), 4-amino-2,2,6,6-tetramethylpiperidine (TEMP, 98 wt%), 4-hydroxy-2,2,6,6-tetramethyl-piperidinooxy (TEMPOL, 98 wt%), 5,5-dimethyl-1-pyrroline N-oxide (DMPO, 97 wt%) and tetracycline (TC) were purchased from Aladdin Chemistry Reagent Co., Ltd. All chemicals were used without further purification, and double distilled (DI) water was used in the experiments.

2.2. Synthesis of MgO₂ nanoparticles and MgNCN/MgO nanocomposites

MgO₂ nanoparticles were synthesized through a facile one-step precipitation method. Typically, 15.38 g of Mg(NO₃)₂·6H₂O was dissolved in 15 mL of distilled water, and then 15 mL of NH₃·H₂O was introduced under constant stirring. After stirring for about 30 min at room temperature (ca. 25 °C), 30 mL of H₂O₂ (30 wt%) was quickly poured into the above mixture and stirred for another 4 h. Subsequently, the precipitate in the solution was collected through filtration and washed several times. Finally, the MgO₂ nanoparticles were obtained after being dried at 70 °C overnight in an oven.

MgNCN/MgO nanocomposites were prepared through calcining the mixture of thiourea and magnesium carbonate basic pentahydrate. Specifically, 1.0 g of thiourea and 0.5 g of (MgCO₃)₄·Mg(OH)₂·5H₂O were mixed and ground thoroughly in an agate mortar. Subsequently, the above mixture was placed into a crucible with a lid wrapped with Al foil, and then calcined at 550 °C for 2 h. After cooling to room temperature, the obtained product was preserved for further use. For comparison, the individual MgNCN and MgO samples were also prepared according to the above procedure, and the detailed description is provided in the ESI.†

2.3. Characterization

The crystal structure of the samples was determined using an X-ray powder diffractometer (Philips X'Pert Pro) using Cu K_α radiation (λ = 1.5418 Å). The XRD patterns were recorded in the range of 10–90°. The functional groups and composition of the samples were determined on a Fourier transform infrared (FTIR) spectrometer (Bruker VERTEX 70) equipped with a KBr beam splitter in the regular scanning region of 4000–400 cm⁻¹. The microstructure and morphology of the prepared samples was observed on a scanning electron microscope (SEM, Nova NanoSEM 450, acceleration voltage 10 kV, working distance 5 mm) and a transmission electron microscope (TEM, Tecnai G2 20, acceleration voltage 200 kV). The Brunauer–Emmett–Teller (BET) surface area was measured on a surface area analyzer (Micromeritics ASAP 2020 M). The valence state and surface composition of the MgNCN/MgO nanocomposites were investigated using an X-ray photoelectron spectrometer (XPS, AXIS-ULTRA DLD-600 W). The concentration of the released Mg²⁺ in solution was determined using an atomic absorption spectrophotometer (AAS, iCE 3000). The NO₂⁻ and NO₃⁻ concentrations were measured on an ion chromatograph (Metrohm 861, Switzerland), and the NH₄⁺ concentration in solution was measured using a water quality analyzer (Huameiwate HM-800).

2.4. Degradation of organic pollutants

MB, OG, AO7 and TC were chosen as target organic pollutants to assess the degradation performance of the MgO₂–MgNCN/MgO system. Typically, 20 mg of MgO₂ nanoparticles was added into 100 mL of organic solution (10 mg L⁻¹) under constant magnetic stirring. After being thoroughly mixed, an appropriate amount of MgNCN/MgO nanocomposites was introduced into the mixture to initiate the degradation reaction. At given time intervals, 3 mL of suspension was extracted and filtered immediately using a 0.22 μm-pore-size membrane filter. The concentration of the residual pollutant in the filtrate was determined using a UV-visible spectrophotometer (UV-670, Shanghai Meipuda Instrument Co., Ltd), and the degradation efficiency was calculated viaeqn (1). To ensure the reliability of the experimental data, all degradation experiments were conducted in duplicate.where C₀ is the initial organic concentration (mg L⁻¹) in solution and C_t is the organic concentration (mg L⁻¹) at time t during the degradation process.

2.5. ROS quenching and electron paramagnetic resonance (EPR) experiments

Various scavengers including IPA, FFA and TEMPOL were employed in the quenching experiments to identify the critical role of the ROS generated during the degradation process. To further determine the dominant ROS generated in the MgO₂–MgNCN/MgO system, the electron paramagnetic resonance (EPR) experiments were conducted on a Bruker EMXmicro-6/1 spectrometer using DMPO and TEMP as spin-trapping agents, respectively. Specifically, 1 mL of the solution was extracted from the degradation system at given time intervals, and mixed with 20 μL of spin-trapping agent immediately. After thoroughly shaking, the mixture was filtered through a 0.22 μm-pore-size membrane filter. Then, a small amount of the filtrate was introduced to the capillary for EPR test. The test parameters were as follows: the microwave power was 2 mW; the microwave frequency was 9.82 GHz; the modulation frequency was 1 GHz; the constant time was 0.1 ms; the scanning time was 80 s.

3. Results and discussion

3.1. Characterization of the MgO₂ nanoparticles and MgNCN/MgO nanocomposites

The phase structure of the as-prepared MgO₂ nanoparticles was identified by the XRD technique. As illustrated in Fig. 1a, three diffraction peaks located at 2θ = 37.1, 53.5 and 63.7° were assigned to the (200), (220) and (311) facets of the MgO₂ phase with a cubic structure (JCPDS 01-076-1363), respectively. No other diffraction peaks related to Mg(OH)₂ or MgO were found in the XRD pattern, suggesting a high purity of the obtained MgO₂ sample. The FTIR spectrum of MgO₂ is presented in Fig. 1b, in which the peaks at 679 and 496 cm⁻¹ correspond to the bending vibration and stretching vibration of the Mg–O bond.³⁹ The adsorption peak appearing at 864 cm⁻¹ was related to the vibration of the O–O bridge in MgO₂.¹⁸ The two adsorption peaks located at 3431 and 1623 cm⁻¹ were attributed to the vibration of the hydroxyl group of surface-adsorbed water.^40,41 As for the peak at 1431 cm⁻¹, it was ascribed to the vibration of the carbonate group resulting from the adsorbed CO₂.⁴² The microstructure and morphology of the as-prepared MgO₂ could be observed in the SEM and TEM images. As shown in Fig. 1c and d, the obtained MgO₂ was mainly composed of numerous nanoparticles with a high dispersibility and the average size was about 48 nm. In addition, the N₂ adsorption/desorption isotherms (Fig. S1a†) show that the MgO₂ nanoparticles exhibited a BET surface area of 455.94 m² g⁻¹ and a pore volume of 0.58 cm³ g⁻¹. By the calculation of the BJH model, the pore size was distributed in the range of 2–20 nm (Fig. S1b†). The small-sized particles with a high surface area could be beneficial to the improvement of the oxidation ability of MgO₂.

Fig. 1 XRD pattern (a), FTIR spectrum (b), SEM image and the corresponding size distribution (inset) (c), and TEM image (d) of the MgO₂ nanoparticles.

The XRD technique was also employed to analyze the crystal phase of the prepared MgNCN/MgO nanocomposites. In Fig. 2a, the five peaks at 36.9, 42.9, 62.3, 74.7 and 78.6° could be well indexed to the (111), (200), (220), (311) and (222) facets of MgO with a periclase-structure (JCPDS 00-004-0829), respectively. The diffraction peaks appearing at 18.8, 34.0, 45.2, 56.1 and 67.3° were assigned to the (003), (012), (015), (110) and (202) facets of rhombohedral-structured MgNCN (JCPDS 00-051-0540), respectively. No diffraction peaks from other impurity phases were detected, indicating that the obtained MgNCN/MgO nanocomposites mainly contained MgNCN and MgO phases.

Fig. 2 XRD pattern (a), FTIR spectrum (b), SEM image and the corresponding size distribution (inset) (c), and TEM image (d) of the MgNCN/MgO nanocomposites.

The FTIR spectrum of the MgNCN/MgO nanocomposites was measured to investigate the chemical structure and functional groups. As shown in Fig. 2b, the peak at 488 cm⁻¹ was assigned to the vibration of the Mg–O bond.³⁹ The characteristic adsorption peaks located at 2170 and 681 cm⁻¹ were ascribed to the stretching and bending vibration of [NCN]²⁻.^43,44 As well known, [NCN]²⁻ possesses two different electronic structural states: a symmetric structure [N=C=N]²⁻(carbodiimide) and an asymmetric structure [N≡C–N]²⁻(cyanamide).^45,46 According to the previous reports,^47,48 it could be inferred that the MgNCN phase was in the form of Mg[N=C=N] in the MgNCN/MgO nanocomposites. Interestingly, the [NCN]²⁻ characteristic peaks in the MgNCN/MgO nanocomposites shifted to a higher wavenumber than those in individual MgNCN and the physical mixture of MgNCN and MgO with a mass ratio of 1 : 1 (Fig. S2†), implying a change in the electron cloud density. This change indicated that there existed a coordination between MgNCN and MgO in the nanocomposites.

The morphology of the MgNCN/MgO nanocomposites was characterized and is displayed in Fig. 2c and d. The SEM image (Fig. 2c) exhibited that the MgNCN/MgO nanocomposites were composed of nanoparticles with an average size of about 56 nm. Similarly, the TEM image (Fig. 2d) also confirmed that the MgNCN/MgO nanocomposites consisted of numerous small flake-like nanoparticles (40–60 nm). The results of the BET test (Fig. S3†) revealed that the nanocomposites had a high specific surface area of 53.11 m² g⁻¹, which could supply plenty of active sites available for the activation reaction.

Further, the bonding effect between MgO and MgNCN in the MgNCN/MgO nanocomposites was investigated by XPS. The survey spectrum (Fig. S4†) clearly displayed the signals of the C, N, O and Mg elements, which were the main constituent atoms of the MgNCN/MgO nanocomposites. The high-resolution spectrum of C 1s (Fig. 3a) could be divided into three peaks. The peak appearing at 285.0 eV was inferred to be the N=C=N bond, and the peak located at 286.4 eV could be attributed to the C–O coordination bond between MgNCN and MgO.^49,50 As for the peak at 288.7 eV, it might be from residual carbonate in the nanocomposites.⁵¹ The N 1s XPS spectrum (Fig. 3b) of the nanocomposites could be separated into two peaks. The strong peak observed at 398.1 eV was assigned to the Mg–N=C bond. The peak with a lower binding energy of 397.3 eV was considered as the Mg–N coordination bond between MgNCN and MgO,^38,52 because the coordination of MgO could increase the electron cloud density around the N atom, resulting in a decrease of the binding energy. Fig. 3c showed that the high-resolution Mg 1s spectrum of the MgNCN/MgO nanocomposites was deconvoluted in three subpeaks. Since the electronegativity of the N atom is weaker than that of the O atom, the lowest binding energy (1302.4 eV) and highest binding energy (1304.1 eV) subpeaks were assigned to the N–Mg–N bond in MgNCN and O–Mg–O bond in MgO, respectively. Meanwhile, the middle subpeak at 1303.2 eV could be ascribed to the Mg–N coordination bond between MgNCN and MgO. The O 1s XPS spectrum (Fig. 3d) was also fitted into three peaks. The peaks at 531.5 and 532.7 eV were related to the lattice oxygen of MgO and adsorbed oxygen, respectively.^53,54 The peak at 529.8 eV might be attributed to the C–O coordination bond between MgNCN and MgO,⁵⁵ which was in good agreement with the above analysis. Therefore, it could be concluded that the Mg–N and C–O coordination bonds existed between MgO and MgNCN in the nanocomposites.

Fig. 3 XPS spectra of C 1s (a), N 1s (b), Mg 1s (c) and O 1s (d) for the MgNCN/MgO nanocomposites.

3.2. Degradation performance of the MgO₂–MgNCN/MgO system

To evaluate the degradation performance of the MgO₂–MgNCN/MgO system, a series of experiments were conducted under different conditions, including the dosages of MgO₂ and the MgNCN/MgO nanocomposites, initial solution pH and degradation temperature. The degradation efficiencies are exhibited in Fig. 4, and the corresponding kinetic fitting results are listed in Fig. S5 and S6.† According to the comparison of the R² values (Table S1†), the MB degradation processes could be well described by the pseudo-first-order kinetic model. Fig. 4a showed that the MB degradation was significantly affected by the MgO₂ dosage. If without MgO₂ addition, there was a slight decrease (9%) in the MB concentration due to the physical adsorption of the MgNCN/MgO nanocomposites. Once MgO₂ was introduced into the degradation system, the degradation efficiency for MB rose rapidly with the increase of the MgO₂ dosage. This point could be also proved by the increase of the k₁ value from 0.120 to 1.120 h⁻¹ as the MgO₂ dosage rose from 5 to 30 mg (Fig. S5a and Table S1†). When the amount of MgO₂ was increased to 20 mg, MB was almost completely degraded within 8 h. Thereafter, by further increasing the MgO₂ dosage, the degradation efficiency was slightly enhanced. Therefore, the dosage of MgO₂ was selected to be 20 mg in the following experiments.

Fig. 4 Effects of the MgO₂ dosage (a), MgNCN/MgO dosage (b), initial solution pH (c) and temperature (d) on MB degradation (reaction conditions:100 mL of 10 mg L⁻¹ MB, 20 mg of MgO₂, 10 mg of MgNCN/MgO).

The degradation performances with different amounts of MgNCN/MgO are displayed in Fig. 4b. Similarly, the MB concentration decreased slightly in the absence of MgNCN/MgO, because of the adsorption of MgO₂. When only 2 mg of MgNCN/MgO was added, the MB degradation efficiency increased sharply to 76%, indicating that MgNCN/MgO played a crucial role in MB degradation. When the dosage of MgNCN/MgO was increased to 10 mg, MB was almost completely degraded within 8 h. The degradation efficiency for MB was hardly enhanced as the MgNCN/MgO dosage was adjusted to 15 mg. Unexpectedly, the degradation efficiency was a little reduced when the amount of the MgNCN/MgO nanocomposites was raised to 20 mg. This might be ascribed to the reason that excess MgNCN/MgO could consume some of the generated ROS, thus hindering MB degradation. Accordingly, the suitable MgNCN/MgO dosage for MB degradation was estimated to be 10 mg under the experimental conditions.

To further confirm the key contribution of the MgNCN/MgO nanocomposites as activators for organic degradation, the degradation performances with individual MgNCN and MgO and the physical mixture of MgNCN and MgO were also investigated under the same conditions. As shown in Fig. S7,† the degradation efficiencies with the physical mixture and individual MgNCN and MgO were no more than 20% due to the adsorption of materials, far inferior to that with the MgNCN/MgO nanocomposites (99.5%). Combining the above characterization and other reports,^56,57 it was concluded that the bonding effect between MgNCN and MgO in the nanocomposites constructed the electron-rich N centers, which played a vital role in activating MgO₂ to generate ROS for MB degradation.

The effect of the initial solution pH was also investigated to evaluate the applicability of the MgO₂–MgNCN/MgO system. Excitingly, the MgO₂–MgNCN/MgO system could effectively degrade MB in a broad pH range from 4 to 12 (Fig. 4c). When the initial solution pH was set at 2, the degradation efficiency was only 4.9% within 8 h. This was due to the fact that both MgNCN/MgO and MgO₂ were almost completely dissolved under strong acidic conditions. When the initial solution pH was adjusted to 4–10, all of the degradation efficiencies reached more than 98% within 8 h. Unexpectedly, the degradation efficiency for MB with a high rate constant of 1.033 h⁻¹ exceeded 99% within 6 h as the initial pH value was tuned to 12 (Fig. S5c and Table S1†). It might be due to the fact that strong alkalinity could inhibit the decomposition of the MgNCN/MgO nanocomposites in solution,⁵⁸ allowing MgO₂ to be effectively activated to generate more ROS for organic degradation.^59,60 To the best of our knowledge, this was the first time to report that a system could effectively degrade organic pollutants under strong alkaline conditions, which made the MgO₂–MgNCN/MgO system far superior to other AOP systems (Table S2†).

As shown in Fig. 4d, the rise of degradation temperature could significantly improve the degradation efficiency for MB in the MgO₂–MgNCN/MgO system. When the temperature was 5 °C, the degradation efficiency for MB was only 18.2% within 8 h. When the temperature was increased to 35 °C, the MB solution could be almost completely degraded within 4 h, with the highest rate constant of 1.368 h⁻¹ (Table S1†). The enhancement effect of temperature could be attributed to the increased average kinetic energy of molecules, which accelerated molecular diffusion and facilitated the interaction of MgO₂ with MgNCN/MgO to yield ROS,¹¹ thus boosting the degradation performance of the MgO₂–MgNCN/MgO system towards organic pollutants.

3.3. Anti-interference and adaptability of the MgO₂–MgNCN/MgO system

Firstly, several common inorganic anions (10 mM Cl⁻, NO₃⁻, SO₄²⁻ and HCO₃⁻, respectively) and natural organic matter (10 mg L⁻¹ HA) were selected as matrix species to investigate their effects on MB degradation in the MgO₂–MgNCN/MgO system. As shown in Fig. 5a, these common matrix substances had negligible influences on the removal of MB in the MgO₂–MgNCN/MgO system. As for HA, the slight inhibitory effect might result from the competitive degradation between MB and HA.⁶¹ Hence, the MgO₂–MgNCN/MgO system exhibited strong resistance to the common matrix species Cl⁻, NO₃⁻, HCO₃⁻, SO₄²⁻ and HA.

Fig. 5 Effects of matrix species 10 mM Cl⁻, NO₃⁻, SO₄²⁻, HCO₃⁻ and 10 mg L⁻¹ HA (a) and water sources (b) on MB degradation and degradation performance of the MgO₂–MgNCN/MgO system towards 10 mg L⁻¹ OG, AO7 and TC (c) (reaction conditions: 100 mL of 10 mg L⁻¹ organic pollutant, 20 mg of MgO₂, 10 mg of MgNCN/MgO).

Secondly, water sources with different water qualities, including tap water (Wuhan city), Yangtze river water (E: 114°17′48′′, N: 30°35′5′′), East lake water (E: 114°22′6′′, N: 30°32′33′′), Han river water (E: 114°14′48′′, N: 30°33′59′′) and Moon lake water (E: 114°15′16′′, N: 30°33′43′′) were utilized to probe the adaptability of the MgO₂–MgNCN/MgO system. Given the existence of organic and inorganic pollutants and microorganisms in tap water and natural water (Tables S3 and S4†), the time of MB degradation with the same dosages was extended to 24 h. It was found that the MgO₂–MgNCN/MgO system still exhibited good degradation performance towards organic pollutants in these water sources, and all the degradation efficiencies for MB reached more than 97% (Fig. 5b).

In addition to cationic dye MB, anionic dyes OG and AO7 and antibiotic TC were also selected to examine the degradation performance of the MgO₂–MgNCN/MgO system towards different types of organic pollutants. As expected, all these organic pollutants could be effectively degraded in the MgO₂–MgNCN/MgO system (Fig. 5c). Moreover, the MgO₂–MgNCN/MgO system also exhibited excellent degradation performance for low-concentration organic pollutants (Fig. S8†), and was expected to be used for the removal of trace organic pollutants. Therefore, it could be considered that the MgO₂–MgNCN/MgO system possessed high adaptability and great potential for the remediation of organic-polluted wastewater.

3.4. Slow-release characteristic for continuous degradation of organic pollutants

The slow-release characteristic of MgO₂ was studied by determining the H₂O₂ concentration in aqueous solution with a standard solution of potassium permanganate (KMnO₄), and the detailed procedures are provided in the ESI.† As shown in Fig. 6a, the H₂O₂ concentration released from MgO₂ gradually increased with time, and the process could be well described by the pseudo-zero-order kinetic model (Fig. S9†). However, the concentration of released H₂O₂ was maintained at a low level. Even after 48 h, the concentration was still less than 0.75 mM. In this case, the release rate of MgO₂ was about 43%, suggesting that most of MgO₂ was not decomposed into H₂O₂. Thus, it was confirmed that MgO₂ had a slow-release characteristic in solution to meet the requirement of relatively long-term oxidation. Meanwhile, the concentrations of the released Mg²⁺ from the MgO₂–MgNCN/MgO system were monitored and are shown in Fig. S10.† Clearly, the Mg²⁺ concentrations in solution were below the limit of the drinking water standard,⁶² suggesting that the leakage of Mg²⁺ from the AOP system had almost no impact on the environment. Subsequently, the continuous degradation ability of the MgO₂–MgNCN/MgO system was evaluated and is presented in Fig. 6b. It could be seen that the degradation efficiency still reached 86.0% after four consecutive runs, which resulted from the slow-release characteristic of MgO₂. As for the significant decrease in the degradation efficiency (42.8%) at the 5th run, it might be attributed to the consumption of most MgO₂ and hydrolysis of MgNCN/MgO after several runs, which could be confirmed by the XRD and FTIR analyses (Fig. S11†).

Fig. 6 Slow release of MgO₂ in solution (200 mg L⁻¹, 25 °C) (a), continuous degradation performance of the MgO₂–MgNCN/MgO system towards MB (b), comparison of degradation performances with MgO₂ and equimolar H₂O₂ before and after exposure to air for 12 h (c) and comparison of oxidant utilization efficiency in different AOP systems (d).

Further, the oxidation ability of MgO₂ was compared to that of equimolar H₂O₂ (30 wt%), under the same number of MgNCN/MgO activation. As could be seen from Fig. 6c, the H₂O₂–MgNCN/MgO system could completely degrade the MB solution in 1.5 h, which was far faster than the MgO₂–MgNCN/MgO system. After being exposed in air for 12 h, liquid H₂O₂ could hardly degrade MB. Meanwhile, MgO₂ maintained almost the same degradation performance towards MB as unexposed MgO₂, even after being exposed in air for 21 d (Fig. S12†). The results could be attributed to the fact that liquid H₂O₂ decomposed easily into water and oxygen after being exposed, while MgO₂ remained almost unchanged. Based on the reported method,⁶³ the utilization efficiency of MgO₂ in this AOP system was also evaluated and compared with common H₂O₂ and CaO₂. Excitingly, the oxidant utilization efficiency in the MgO₂–MgNCN/MgO system (6.87%) was significantly higher than that in other AOP systems (Fig. 6d). Considering the long-term demands of water treatment, the continuous degradation ability, high stability and utilization efficiency of the MgO₂–MgNCN/MgO system make it more advantageous than other AOP systems.

3.5. Degradation mechanism and process

The organic degradation in AOP systems is highly dependent on the ROS generated during the degradation process. To explore the type of ROS generated in the MgO₂–MgNCN/MgO system, a series of quenching experiments were conducted using IPA, TEMPOL and FFA as scavengers for ˙OH, O₂˙⁻ and ¹O₂, respectively. As illustrated in Fig. 7a, IPA had a weak significant inhibitory effect on MB degradation in the MgO₂–MgNCN/MgO system. Even when the concentration of IPA was increased to 1000 mM, the degradation efficiency of MB still reached about 84%, suggesting the minor contribution of ˙OH to MB degradation. Similarly, the addition of different amounts of TEMPOL had almost no effect on MB degradation (Fig. 7b), implying that O₂˙⁻ might not be produced in the degradation system. In contrast, FFA had a significant inhibitory effect on MB degradation. The degradation efficiency of MB gradually decreased as the concentration of FFA rose (Fig. 7c). When the concentration of FFA was increased to 10 mM, the MB degradation reaction was nearly completely suppressed. Thus, non-radical ¹O₂ might be generated in the MgO₂–MgNCN/MgO system, which played a critical role in the degradation of organic contaminants.

Fig. 7 MB degradation with the addition of IPA (a), TEMPOL (b), and FFA (c) and EPR spectra for detecting the ROS in the MgO₂–MgNCN/MgO system (d).

EPR tests were carried out to identify the dominant ROS generated in the AOP system. As could be seen in Fig. 7d, a weak signal related to the DMPO–˙OH adduct, typical fourfold peaks (the intensity ratio of 1 : 2 : 2 : 1) with hyperfine coupling of a_N = a_H = 14.9 G,^64,65 appeared in the EPR spectrum, demonstrating very little ˙OH generation in the system. However, the characteristic signal of the DMPO–O₂˙⁻ adducts was not detected in this AOP system. In contrast, the characteristic 1 : 1 : 1 triplet signal of TEMP–¹O₂ was clearly observed, further confirming the generation of ¹O₂ in this system, which was consistent with the results of the quenching experiments. Thus, it was safely concluded that the generated ¹O₂ in the MgO₂–MgNCN/MgO system was the dominant ROS responsible for MB degradation via a non-radical process.

The intermediates and residues during the degradation reaction were analyzed through a variety of techniques to reveal the degradation process. The ion chromatograph and water quality analyzer were used to measure the concentrations of NO₂⁻, NO₃⁻ and NH₄⁺ generated in the MgO₂–MgNCN/MgO–H₂O and MgO₂–MgNCN/MgO–MB systems, respectively. As could be seen in Fig. 8a and b, the concentrations of NO₂⁻ and NO₃⁻ in the MgO₂–MgNCN/MgO–MB system were significantly lower than those in the MgO₂–MgNCN/MgO–H₂O system. Meanwhile, the MgO₂–MgNCN/MgO–MB system possessed a higher NH₄⁺ concentration than the MgO₂–MgNCN/MgO–H₂O system. The high NH₄⁺ concentration in the MgO₂–MgNCN/MgO–MB system was ascribed to the fact that NH₄⁺ not only originated from the decomposition of MgNCN/MgO,^58,66 but also stemmed from the degradation of MB.^67,68 Correspondingly, the low NO₂⁻ and NO₃⁻ concentrations meant that the generated ROS was for preferentially degrading MB rather than oxidizing NH₄⁺. Combining the above results with other reports,^69,70 the degradation process could be speculated as follows: (i) MgO₂ interacted with MgNCN/MgO to generate ROS (¹O₂, ˙OH) and NH₄⁺; (ii) the generated ROS degraded MB to produce CO₂, H₂O and NH₄⁺; (iii) NH₄⁺ was oxidized to NO₂⁻ by the ROS; (iv) NO₂⁻ was further oxidized by the ROS to NO₃⁻.

Fig. 8 Ion chromatogram spectra of NO₂⁻ and NO₃⁻ (a), the concentrations of NO₂⁻, NO₃⁻ and NH₄⁺ (b), and XRD patterns (c) and FTIR spectra (d) of the residues in different systems for 12 hours.

The XRD patterns and FTIR spectra of the solid residues in the two systems were also analyzed to investigate the conversion products of MgO₂ and MgNCN/MgO. As shown in Fig. 8c, all of the diffraction peaks in the XRD patterns could be well matched to the hexagonal-structured Mg(OH)₂ (JCPDS 00-044-1482), which came from the hydrolysis of MgO₂ and MgNCN/MgO. In addition, it was observed that the peak intensities of the residue in the MgO₂–MgNCN/MgO–MB system were lower than those in the MgO₂–MgNCN/MgO–H₂O system, which was attributed to the reduction of Mg(OH)₂ crystallinity. This phenomenon might be caused by the adsorption of the degradation intermediates from MB on the generated Mg(OH)₂ surface.

In the FTIR spectra of the solid residues (Fig. 8d), the peaks at 3699 and 455 cm⁻¹ were assigned to the O–H and Mg–O bond vibration of Mg(OH)₂.⁷¹ The weak peak at around 680 cm⁻¹ was related to the bending vibration of the remaining [NCN]²⁻.^43,72 The peaks appearing at 3434, 1633 and 1429 cm⁻¹ correspond to the vibrations of adsorbed H₂O and CO₂.^40,42 The results also demonstrated that the obtained solid residues were mainly Mg(OH)₂. Additionally, the intensity of the peak at 3699 cm⁻¹ in the MgO₂–MgNCN/MgO–MB system was relatively weaker than that in the MgO₂–MgNCN/MgO–H₂O system, while the peak at around 680 cm⁻¹ was comparatively stronger than that in the MgO₂–MgNCN/MgO–H₂O system. These implied that the decomposition of the MgNCN/MgO nanocomposites has been delayed, which further confirmed that the generated ROS preferentially degraded organic molecules. According to the above results, the process of organic degradation in the MgO₂–MgNCN/MgO system was proposed and is briefly described in Fig. 9.

Fig. 9 Schematic illustration of organic degradation in the MgO₂–MgNCN/MgO system.

4. Conclusions

In summary, this study demonstrated a brand-new AOP system consisting of MgO₂ nanoparticles and MgNCN/MgO nanocomposites, which were both synthesized by a simple one-step method. The novel system could efficiently degrade organic contaminants in a wide pH range (4–12), and was rarely affected by common matrix species and water sources. It was believed that the bonding effect between MgNCN and MgO in the MgNCN/MgO composites played a vital role in activating MgO₂ to generate singlet oxygen (¹O₂), which was mainly responsible for the degradation of organic contaminants. The presented system possessed a long-term oxidation ability for consecutive degradation of organic pollutants in wastewater, due to the high stability and slow-release characteristic of MgO₂. With the merits of environmental friendliness, a simple preparation procedure, wide application ranges (pH range and different water qualities and types of organic pollutants), a slow-release characteristic and long-term oxidation, the MgO₂–MgNCN/MgO system is an extremely attractive candidate for the treatment of organic pollutants in wastewater.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The authors acknowledge the experimental help from the Huazhong University of Science & Technology Analytical and Testing Center. Prof. X. Wang was supported by the National Science Foundation for Distinguished Young Scholars of China (No. 51725504). Prof. P. K. Wong was supported by the National Natural Science Foundation of China (No. 42077333).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1en00751c

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