Nano silver oxide-modified activated carbon as a novel catalyst for efficient removal of bacteria and micropollutants in aquatic environment

Abstract

Heterogeneous Fenton process is a promising water treatment technology for sterilization and degradation of organic pollutants, due to the strong oxidation of hydroxyl radicals (OH˙) generated. However, the low H₂O₂ activation efficiency and the instability of catalyst leading to low OH˙ production restricted development of this technology. Herein, we synthesized a novel porous activated carbon-loaded nano silver oxide (nAg₂O/AC) catalyst to enhance the activation of H₂O₂ for removing bacteria (E. coli) and micropollutants (Tetracycline, TC) from water. In the nAg₂O/AC Fenton system, reductive hydroxyl groups on AC accelerated Ag(I)/Ag cycle through mediated electron transfer, which markedly increased H₂O₂ activation efficiency to 73.7% (About 2.9 times that of traditional Fenton). Hence, nAg₂O/AC Fenton achieved up to 6.0 log and 100% removal efficiency for E. coli and TC, respectively. The OH˙ as the major oxidizing species in nAg₂O/AC Fenton system was detected and verified by radical scavenging tests and electron spin resonance (ESR) measurement. After 4 and 5 cycles of experiments, the removal of E. coli and TC still reached 5.2 log and 96%, respectively, confirming good stability of nAg₂O/AC for considerable application prospects. This study concluded that nAg₂O/AC is a promising H₂O₂ catalyst for simultaneous removal of bacteria and micropollutants in aqueous environment.

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1 Introduction

Water environment security is closely related to social development and human health. With the advancement of technology and the widespread use of antibiotics, antibiotic resistant bacteria and trace pollutants have been frequently detected in various water bodies,¹ posing a serious threat to aquatic ecology and human health. Traditional chlorination method for treating such bacteria is becoming increasingly inadequate,² and even worse, it could react with micropollutants to generate toxic byproducts (e.g., halogenated organic compounds and chlorate) resulting in secondary contamination.³ Instead, various emerging technologies including membrane filtration,^4,5 chemical reduction/oxidation,^6–8 electrochemistry⁹ and photocatalytic degradation^10–13 have been studied for removing bacteria and micropollutants. Despite some progress in experiments, the practical application of these technologies is still constrained by high energy consumption, low efficiency, and complex operating conditions.

Advanced oxidation processes (AOPs) are recognized as a promising water purification technology,¹⁴ owing to the high-efficiency sterilization and degradation effect of strong oxidizing reactive oxygen species (ROS) (e.g., OH˙ and O₂˙⁻) produced. ¹⁵ It is well known that Fenton reaction is one of the most efficient method in AOPs, which can produce abundant OH˙ through Fe²⁺/H₂O₂ reaction. ¹⁶ Nevertheless, there are still some defects in Fenton reaction such as the low activation efficiency of H₂O₂, restriction of pH application range (2.0–4.0) and massive ferric salt precipitation as solid waste.¹⁷ Given this, heterogeneous Fenton catalysts are developed and applied,¹⁸ especially various nanometal oxides with natural antibacterial property and potential catalytic activity.¹⁹

Nano silver oxide (nAg₂O) has better sterilization and catalytic performance compared to other metal oxides, such as Fe₂O₃, Co₂O₃, CuO and MoO₂.²⁰ However, when nAg₂O is used alone, its activity is still low and is greatly affected by agglomeration.²¹ Activated carbon (AC) is a satisfactory alternative carrier for nanoparticles due to its excellent conductivity and large surface area. Our previous work showed that loading nAg₂O onto AC can effectively improve its dispersibility and reactivity, and achieve effective sterilization by activating dissolved oxygen to generate superoxide radical at a wide pH range.²² This means that nAg₂O/AC, if used as a heterogeneous catalyst, would have enormous potential to activate hydrogen peroxide (H₂O₂). As a common green oxidant in water treatment, H₂O₂ is more easily activated to produce stronger oxidizing hydroxyl radicals (OH˙) which possess better sterilization and organic matter degradation effects.²³ In addition, due to its stability and activity over a wide pH range, nAg₂O/AC as a heterogeneous catalyst is expected to improve the decomposition efficiency of H₂O₂ producing more OH˙, ultimately achieving efficient removal of bacteria and organic micropollutants in water.

In this work, nAg₂O/AC as a novel heterogeneous Fenton catalyst is developed to enhance H₂O₂ activation efficiency for more OH˙ radicals' production to remove bacteria and micropollutants in water. The nAg₂O/AC catalyst was synthesized by a simple hydrothermal method and applied for H₂O₂ activation to treat wastewater. Escherichia coli (E. coli) and tetracycline (TC) were regarded as the target bacteria and micropollutants, respectively. A series of characterization techniques were used to investigate surface characteristics of catalysts and further explore electron transfer process between nAg₂O and AC. The effects of initial pH in Fenton process were investigated in the range of pH 5.0–9.0. Subsequently, cycle and regeneration tests were carried out for estimating the useful life of nAg₂O/AC during treatment. Mechanism of nAg₂O/AC Fenton process was explored through quenching experiments and ESR measurement. In addition, the decomposition rate of H₂O₂ was further quantitatively analyzed by fluorescence spectroscopy. Consequently, the obtained findings could be useful in providing significant insight into the development of novel heterogeneous Fenton catalysts and their application in the treatment of composite pollutants in water.

2 Materials and methods

2.1 Materials and chemicals

Details of chemicals and materials are provided in Text S1 of the ESI.†

2.2 Preparation of catalysts

Based on our previous studies of modifying activated carbon with metal oxides,²⁴ a facile impregnation-calcination method was applied to prepare the nAg₂O/AC catalyst. Briefly, 316 mg AgNO₃ and 200 mg PVP was mixed evenly in 200 mL deionized water. Subsequently, 20.0 g activated carbon was poured into and mixed evenly. After 12 h of magnetic stirring, the beaker was allowed to stand for 24 h. After being filtered and washed to neutral with pure water, the remaining solid was placed at 75 °C for 12 h to obtain nAg₂O/AC catalyst.

To measure the silver-loading rate, the nAg₂O/AC was treated with nitric acid for 8 h to elute silver and the silver concentration was measured by an atomic absorption spectrometer (AAS). The silver-loading rate was determined as ±1.5%. Unsupported nAg₂O was synthesized as a control. Briefly, under magnetic stirring, 1.0 g of AgNO₃ was dissolved in 100 mL deionized water in a conical flask. The pH value was adjusted to 12 by 1 M NaOH. The nAg₂O was achieved by filtering and drying the residue at 70 °C for 24 h.

2.3 Experiments

Batch experiments were performed in 200 mL conical flasks. Each flask contained 100 mL bacteria (10⁶ CFU mL⁻¹ E. coli) and micropollutants (1 mg L⁻¹ TC), in which the preparation of E. coli solutions is detailed in Text S2.† After injecting 0.1 g nAg₂O/AC and 3.2 mM H₂O₂, the flask was sealed with parafilm and shaken at 30 °C to start nAg₂O/AC-H₂O₂ heterogeneous Fenton reaction for removing bacteria and micropollutants. As the reaction lasted for 1, 3, 5, 10 and 15 min respectively, 1 mL solution was collected to detect TC concentration. Meanwhile, 0.1 mL solution was taken out to calculate bacterial concentration by plate counting method. The residual E. coli density was calculated by the number of colonies and utilized to evaluate the inactivation efficiency ²⁵ and the detailed identification process is described in Text S2.† Similarly, as control experiments, the nAg₂O/H₂O₂, AC/H₂O₂, nAg₂O/AC alone and H₂O₂ alone systems were also carried out according to the above procedures, respectively. After reaction, silver ions (Ag⁺) in water were detected.

Cycle and regeneration tests were conducted to evaluate stability and reusability of the catalysts.²⁶ The first cycle reaction of nAg₂O/AC catalyst was similar to the heterogeneous Fenton process described above. During reaction, removal efficiency and leached Ag⁺ were measured. The first experiment ended and the nAg₂O/AC catalyst was recovered through filtration and drying. Immediately, recycled nAg₂O/AC was applied for 5 cyclic tests as described in the first process. After cycle tests, using nitric acid to remove silver on activated carbon, and regained activated carbon was used for synthesizing catalysts based on the previous synthesis method. The regenerated nAg₂O/AC was then used in heterogeneous Fenton process for removing pollutants to evaluate reusability.

2.4 Effects of scavengers and detection of OH˙

The methods of detection in this study are shown in Text S3 of the ESI.†

2.5 Analytical methods

Scanning electron microscope (SEM, Hitachi S-4500, Japan) was used to characterize microstructure of nAg₂O/AC catalyst before and after reaction. The crystal phase on catalyst surface was determined by X-ray diffraction (XRD, D8 ADVANCE, Bruker, Germany) with the Cu-Kα radiation source. Besides, An X-ray photoelectron spectroscopy (XPS, K-Alpha, USA) with a Ka-Al radiation was applied to analyze the valence change of different elements in nAg₂O/AC. To verify free radicals produced in these Fenton systems, electron spin resonance spectrometer (ESR, Bruker EMX A300-10/12, Germany) was applied with 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) as spin-trapping agents.²⁷ Flame atomic absorption spectrophotometer (FAAS, HITACHI Z-2000, Japan) was utilized to identify the concentration of Ag⁺ in solution.

The residual E. coli density (log CFU mL⁻¹) was calculated according to eqn (1).

E. coli density = log₁₀C (1)

where C is the bacterial density at t min.

The Ag leaching rate in nAg₂O/AC was counted using eqn (2).

Leaching rate = (leached Ag)/(loaded Ag) × 100% (2)

3 Results and discussion

3.1 Characterization of catalysts

A comprehensive study was conducted on the physicochemical properties and changes of nAg₂O/AC catalysts before and after reaction in the nAg₂O/AC-H₂O₂ system, providing basic support for revealing the reaction mechanism of nAg₂O/AC activating H₂O₂.

X-ray diffraction analysis (Fig. 1a) was performed on nAg₂O/AC. The nAg₂O/AC diffraction peaks at 26.8°, 32.7°, 38.2°, 55.3°, 65.5° before reaction corresponded to (110), (111), (200), (220), and (311) crystal planes of the Ag₂O standard diffraction card (JCPDF card no.75-1532), ²⁸ respectively. After the reaction, these diffraction peaks showed certain enhancement, weakening, or disappearance. For example, the peak at 38.1° weakened, while the peak at 65.6° disappeared. This phenomenon may be caused by the reduction of catalyst particles or phase transformation during the reaction process. In addition, the characteristic diffraction peaks of AC at 25° and 44° showed some enhancement after the reaction, indicating that the carbon matrix of the catalyst was partially exposed after reaction.

Fig. 1 (a) XRD patterns and (b) FTIR spectra of nAg₂O/AC in reaction.

Composition and changes of nAg₂O/AC functional groups before and after the reaction were analyzed by FTIR, as shown in Fig. 1b. Stretching vibration characteristic peaks of O–H, C–H, C=O, C=C, C–C and C–O appeared respectively at frequency bands 3420, 2817, 1735, 1519, 1360 and 938 cm⁻¹ of nAg₂O/AC before reaction.²⁹ In addition, characteristic absorption peaks at 502 and 613 cm⁻¹ corresponded to the stretching vibration of Ag–O.³⁰ Compared with that before reaction, many vibration peaks of nAg₂O/AC appeared to be strengthened, weakened, disappeared, or shifted after the reaction. For example, the C=O vibration peak at 1735 cm⁻¹ and C–C vibration peak at 1360 cm⁻¹ increased, the O–H vibration peak at 3420 cm⁻¹ decreased, and the C–H vibration peak at 2817 cm⁻¹ and Ag–O vibration peak at 502 and 613 cm⁻¹ shifted. These phenomena indicated that nAg₂O existed in the modified AC, and the functional group type of AC surface had changed significantly during reaction, which meant that there may be a strong redox reaction between nAg₂O/AC and H₂O₂.

Surface structures and composition of nAg₂O/AC during heterogeneous Fenton progress were measured as Fig. 2 presented. The surface of fresh AC had abundant and irregular pores, which was conducive to its modification by nAg₂O particles. After AC was modified, some nano-sized aggregates were evenly distributed on its surfaces and pores and these aggregates were probably ascribed to nAg₂O. According to EDS analysis in Table S1,† nAg₂O/AC contained 1.5% Ag compared to fresh AC, implying the successful loading of nAg₂O on AC surface. In addition, EDS mapping images also showed that silver was effectively and evenly loaded on AC. The above results were also in agreement with XRD and FTIR results. After Fenton reaction, there was little change in the microstructure of nAg₂O/AC and the silver content lost slightly by about 0.8%. These results indicated that the binding of nAg₂O/AC composite materials was relatively stable before and after the reaction, providing reliable theoretical data support for the subsequent cycling and stability of the catalyst.

Fig. 2 (a–e) SEM-EDS patterns of nAg₂O/AC before and after reaction.

As illustrated in Fig. 3, X-ray photoelectron spectra (XPS) was used for elemental analysis. Fig. 3a displayed minimal changes of Ag, O and C peaks before and after reaction. This suggested stability of catalyst, corresponding to previous XRD and SEM analysis. The Ag 3d XPS spectrum of nAg₂O/AC (Fig. 3b) exhibited two major peaks at 374.2 and 368.1 eV, respectively, which could be fitted with two components.³¹ The diffraction peaks at 374.6 and 368.6 eV were attributed to Ag, whereas the peaks centered at 373.9 and 368.9 eV were assigned to Ag(I).³² According to peak intensity, the content of Ag(I) and Ag in nAg₂O/AC before reaction was equivalent, but Ag(I) was significantly higher than Ag after the reaction. This phenomenon indicated that nAg₂O/AC occurred an oxidation–reduction reaction during the activation of H₂O₂, leading to the oxidation of Ag to Ag(I). Fig. 3c and d depict the high-resolution spectra of nAg₂O/AC before and after reaction, and the composition of each functional group is obtained through peak fitting. The peak intensity of each functional group showed different levels of enhancement or weakening. For example, the C–OH peak exhibited a significant decrease during reaction process, while the C–C and C–O peaks increased after the reaction. This may be attributed to the oxidation of reducing C–OH group during reaction, which may be accompanied by the reduction reaction of Ag(I) to Ag. This phenomenon was consistent with the formation of Ag in Fig. 3b and consistent with the changes in various functional groups in the previous FTIR analysis. Due to the ability of Ag to activate H₂O₂ to produce strong oxidizing OH˙, the changes in functional groups and elemental states mentioned above are of great significance for the decontamination performance of nAg₂O/AC-H₂O₂ systems in water.

Fig. 3 XPS survey of (a) nAg₂O/AC, high-resolution XPS survey of (b) Ag 3d, (c) C 1s and (d) O 1s.

3.2 Removal performance

To investigate the enhanced removal performance of nAg₂O/AC-H₂O₂ heterogeneous Fenton system for E. coli and TC, it was compared with control systems such as nAg₂O/AC system, H₂O₂ system, AC-H₂O₂ system, and nAg₂O–H₂O₂ system, as illustrated in Fig. 4a and c. From the results, it can be seen that compared to the bacterial density under natural conditions (5.8 log), the H₂O₂ system, AC-H₂O₂ system, and nAg₂O–H₂O₂ system only reduced the bacterial density to 5.2, 5.1, and 4.6 log after 15 min of reaction, indicating that the oxidative sterilization ability of H₂O₂ was weak, and AC had almost no activation effect on H₂O₂, while nAg₂O had very limited activation ability on H₂O₂. Although the nAg₂O/AC system exhibited a slightly stronger sterilization effect, it only dropped to 4.1 log within 15 min. As mentioned in the previous work, it took a long time (about 3 hours) for nAg₂O/AC to achieve good sterilization effect, making it difficult to achieve satisfactory sterilization effect in a short period of time. However, once nAg₂O/AC and H₂O₂ participated in reaction together, cell density decreased rapidly and was close to 0 log at 10 min. The high inactivation efficiency displayed in nAg₂O/AC Fenton system also confirmed the excellent catalytic performance of nAg₂O/AC for H₂O₂. Similar removal results (Fig. 4c) can be obtained in the treatment of TC micropollutants. Compared with the control systems, the nAg₂O/AC Fenton system rapidly and completely degraded TC micropollutants within five minutes, indicating that the nAg₂O/AC Fenton system had excellent removal performance for both bacteria and organic micropollutants, which is of great significance for the treatment of multiplex pollutants in actual wastewater.

Fig. 4 Removal of (a) E. coli and (c) TC in different reaction systems; effect of H₂O₂ dosage on the removal of (b) E. coli and (d) TC in nAg₂O/AC Fenton system. Reaction conditions: 1 g L⁻¹ nAg₂O/AC dosage, 4 mM H₂O₂, 10⁶ CFU mL⁻¹ E. coli, 1 mg L⁻¹ TC, initial pH 7.0.

Dose of H₂O₂ exhibited a significant impact on sterilization efficiency as illustrated in Fig. 4b and d. Within a certain range, as the dosage of H₂O₂ increased, the sterilization efficiency also increased. However, increasing the dosage beyond a certain concentration led to a decrease in sterilization efficiency. When the dosage of H₂O₂ was 1, 2, and 4 mM, respectively, the bacterial survival density of the corresponding system decreased to 2.9, 1.2, and 0 log after 15 min of reaction, indicating that increasing the dosage of H₂O₂ within this range would bring more reactive oxygen species, thereby significantly improving sterilization efficiency. However, when the dosage of H₂O₂ further increased to 8 mM, the sterilization rate and efficiency of the system decreased significantly, indicating that H₂O₂ was already in excess and could undergo scavenging reactions with reactive oxygen species (see eqn (3) and (4)),³³ resulting in a decrease in the effective free radicals available for attacking bacteria. At the same time, effect of H₂O₂ dosage on TC removal exhibited a consistent trend, so an appropriate oxidant dosage was crucial for reaction efficiency. In addition, increasing the amount of H₂O₂ used would also result in higher reagent costs. Considering efficiency and cost, the appropriate dosage of 4 mM H₂O₂ is recommended.

The nAg₂O/AC catalyst is also competitive compared with the previously reported Fenton-like catalyst, both in terms of removal efficiency and rate (Tables S2–S4†). These comparisons suggested the great potential of nAg₂O/AC in activating H₂O₂ for simultaneous removal of bacteria and micropollutions.

The composition of real water environment is very complex, and various anions and cations contained inside may have important impact on nAg₂O/AC Fenton reaction process. Therefore, the influence of common ions (cations such as K⁺, Na⁺, Ca²⁺, Mg²⁺, and anions such as Cl⁻, SO₄²⁻, NO₃⁻, HCO₃⁻) in water on the system was investigated as shown in Fig. 5.

Fig. 5 Effect of coexisting ions (a and b) cations and (c and d) anions and (e and f) NOM in nAg₂O/AC Fenton system. Reaction conditions: 1 g L⁻¹ nAg₂O/AC dosage, 4 mM H₂O₂, 10⁶ CFU mL⁻¹ E. coli, 1 mg L⁻¹ TC, initial pH 7.0, temperature 30 °C.

Fig. 5a and b showed that cations in water generally had a slight impact on removal efficiency, with K⁺ and Na⁺ having almost no effect on sterilization process of the system, while Ca²⁺ and Mg²⁺ had only a small impact, possibly due to their participation in the competition of catalyst surface sites or their susceptibility to hydrolysis disrupting acid–base balance. ³⁴ In contrast, anions had a more significant impact on the reaction process. As shown in Fig. 5c and d, when Cl⁻ and SO₄²⁻ coexisted in the nAg₂O/AC Fenton system, the bacterial survival density significantly increased from 0 to 3.2 and 2.6 log CFU mL⁻¹, respectively, and the residual ratio of TC also increased from 0 to 0.42 and 0.31. This phenomenon may be attributed to the easier complexation reaction between Cl⁻ and SO₄²⁻ with silver ions to generate stable precipitated compounds, which may significantly inhibit the reaction activity of silver, reduce reaction sites on catalyst surface,³⁵ and ultimately lead to a marked decline in its ability to activate H₂O₂. The coexisting HCO₃⁻ had little effect on removal process, while NO₃⁻ had almost no effect on removal reaction.

Natural organic matter (NOM) is another important component of water environment, and the presence of NOM usually has a certain impact on reactions in water, especially on the redox reactions involving reactive oxygen species. As shown in Fig. 5e and f, NOM in water displayed a significant impact on decontamination process of the system. When the NOM concentration was 1 and 3 mg L⁻¹, the bacterial survival density significantly increased from 0 to 1.2 and 2.5 log, and the TC ratio also increased from 0 to 0.18 and 0.43. There may be two reasons for this phenomenon. On the one hand, NOM, as an organic substance, may consume some reactive oxygen species such as ˙OH in the reaction system, resulting in a decrease in the free radicals used to attack contaminant.³⁶ On the other hand, NOM may form a complex with silver ions on catalyst surface, which hindered its contact with H₂O₂ and reduced the activation efficiency. While the concentration of NOM is 5 mg L⁻¹, the bacterial density decreased to a certain extent, indicating that NOM played a promoting role at this time. Some reports suggest that NOM can act as an electron donor to bind to catalyst at a certain concentration, thereby promoting catalytic redox reactions through electron transfer. ³⁷

3.3 Stability evaluation of catalyst

Solution pH is one of the most important factors for catalyst stability in Fenton (like) reaction. Thus, effects of initial pH (5–9) on removal of bacteria and TC were conducted. As depicted in Fig. 6a and b, removal process of bacteria and TC under weakly acidic or neutral conditions presented higher efficiency than that under weakly alkaline conditions. Bacterial inactivation efficiency at pH 5–7 all reached more 5.9 log, which was much higher than 4.5 log at pH 8 and 3.4 log at pH 9. And the removal rate of TC also decreased from 100% at pH 5–7 to 87% at pH 8 and 75% at pH 9. Generally, solution pH value could influence decomposition of H₂O₂ and chemical property of nAg₂O/AC catalyst. It was reported that H₂O₂ is more easily decomposed into OH˙ under acidic conditions as shown in the following eqn (5).³⁸ Thus, the generated OH˙ significantly enhance removal efficiency of bacteria and TC. Conversely, hydroxyl ions (OH⁻) in alkaline solution consumed hydrogen ions (H⁺) and inhibited the production of OH˙, eventually leading to a remarkable decrease of removal efficiency. On the other hand, with the continuous increase of pH value, a great quantity of OH⁻ interacted with Ag(I) in nAg₂O/AC to form stable compounds (see eqn (6) and (7)), ³⁹ consequently reducing the reactivity of this catalyst on activating H₂O₂.⁴⁰

Mⁿ⁺ + H₂O₂ + H⁺ → M⁽ⁿ⁺¹⁾⁺ + ˙OH + H₂O (5)

Ag(I) + OH⁻ → AgOH (6)

2AgOH → Ag₂O + H₂O (7)

Fig. 6 Effect of initial pH on removal of (a) E. coli and (b) TC; (c and d) reusability of nAg₂O/AC in Fenton system. Reaction conditions: 1 g L⁻¹ nAg₂O/AC dosage, 4 mM H₂O₂, 10⁶ CFU mL⁻¹ E. coli, 1 mg L⁻¹ TC, initial pH 7.0, temperature 30 °C.

Additionally, silver leaching from nAg₂O/AC also was investigated as displayed in Fig. S1.† It showed an ultra-low Ag dissolution rate of nAg₂O/AC below 0.6% at pH 5–9, suggesting its stability within this pH scope. The Ag⁺ concentrations in solution between 0.021 to 0.046 mg L⁻¹ also met drinking water safety standards (<0.05 mg L⁻¹). In summary, this nAg₂O/AC catalyst successfully broaden the application range of pH to about neutral in Fenton-like reaction, which is very promising in practical water treatment.

To investigate stability of nAg₂O/AC, circular experiment on removal of bacteria and TC was conducted. As observed from Fig. 6c and d, the high inactivation efficiency exceeding 5.2 log could be obtained in the first four cycle tests, and the removal rate of TC could reach over 96% in five cycles tests, indicating the stability of nAg₂O/AC for efficient catalysis of H₂O₂. Based on the previous XRD and SEM results, there was no significant change in the crystal structure and morphology of nAg₂O/AC before and after reaction, and there was only a slight loss of silver content. These characterization results also confirmed the stability of nAg₂O/AC materials. In addition, the extremely low Ag leaching concentration (<0.035 mg L⁻¹) and leaching rate (<0.3%) directly confirmed the stability of catalyst in Fig. S2.† Starting from the fifth cycle, the sterilization effect of the nAg₂O/AC-H₂O₂ system showed a downward trend, which may be attributed to the continuous consumption of highly reactive zero valent Ag in the cyclic reaction (XPS results in Fig. 3 showed a decrease in Ag content and an increase in Ag(I) content after the reaction), resulting in a decrease in the amount of OH˙ produced by activated H₂O₂. On the other hand, the characteristic peaks of silver in nAg₂O/AC (Fig. 1 and 3) showed no significant changes in intensity in XPS and XRD after the reaction, indicating the stability of the catalyst in cyclic reactions.

After circular experiment, the remaining catalyst solid was cleaned with nitric acid and the obtained activated carbon was used to regenerate catalyst through previous synthesis progress. High removal efficiency of bacteria (5.8 log) and TC (100%) was obtained through the regenerated nAg₂O/AC. This proved the excellent renewability of nAg₂O/AC, implying its potential in practical application.

3.4 Mechanism investigation

The above-mentioned tests and analysis indicated that reactive oxygen species (ROS) generated from nAg₂O/AC heterogeneous Fenton system played an important role in removing bacteria and TC. To identify the contribution of different ROS during reactions, a series of scavenging investigations were carried out with tertbutyl alcohol (TBA), p-benzoquinone (BQ) and catalase as the scavenger for OH˙, O₂˙⁻ and H₂O₂, respectively.⁴¹ As presented in Fig. 7a and b, when no scavenger was involved in nAg₂O/AC heterogeneous Fenton system, the concentration of bacteria and TC decreased rapidly to 0 within 15 min and 10 min, respectively. Similar reaction trends and results could be observed in the presence of BQ and catalase respectively, indicating that effects of O₂˙⁻ and H₂O₂ on removal process was minimal or even negligible owing to the low yield of O₂˙⁻ and the limited inactivation capacity of H₂O₂.⁴² However, after adding TBA into the reaction system, the concentration of bacteria and TC exhibited a significant increase due to the scavenging effect of TBA on OH˙ radicals. Bacteria density and TC proportion rose dramatically from 0 to 2.5 log and 0.29 when TBA was 0.5 mM in solution. After adjusting the TBA to 1.0 mM in solution, the bacteria density and TC proportion further increased to 5.2 log and 0.70, and then remained basically unchanged even increasing TBA to 2.0 mM since all OH˙ radicals had been scavenged. These results confirmed that the OH˙ radicals generated from nAg₂O/AC Fenton system was the major role in removal process and its quantity directly determined the sterilization efficiency.

Fig. 7 Effects of various radical scavengers on removal of (a) E. coli and (b) TC; (c) ESR spectra for the detection of OH˙ signal in the presence of 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO), (d) the fluorescence spectrum of terephthalic acid (TA) mixed with different reaction systems after reaction for 5 min. Reaction conditions: 1 g L⁻¹ nAg₂O/AC dosage, 4 mM H₂O₂, 10⁶ CFU mL⁻¹ E. coli, 1 mg L⁻¹ TC, 1 mM BQ, 1 mM catalase, initial pH 7.0, temperature 30 °C.

To directly detect the generated OH˙ radicals in reaction, electron spin resonance (ESR) technique was conducted with DMPO as the spin-trapping agents.²⁷ Fig. 7c displayed that no radical signal was detected in nAg₂O/AC alone system. Slightly different from this, some weak DMPO-OH˙ signals appeared in H₂O₂ alone system, implying that a small amount of H₂O₂ could be decomposed into OH˙ radicals. However, strong DMPO-OH˙ signals characterized by a typical 4-fold peak with an intensity ratio of 1 : 2 : 2 : 1 could be clearly observed in nAg₂O/AC Fenton system. It confirmed that H₂O₂ could be efficiently catalyzed by nAg₂O/AC to produce abundant OH˙ radicals. The results were in agreement with the radical scavenging tests (see Fig. 7a and b), in which OH˙ radicals were the major ROS in nAg₂O/AC Fenton reaction.

Subsequently, A quantitative analysis towards the amount of OH˙ radicals generated from different systems was carried out with fluorescence spectrum. Here, non-fluorescent terephthalic acid (TA) was used to trap OH˙ radicals to produce strongly fluorescent hydroxyterephthalic acid (HTA).²⁷ As depicted in Fig. 7d, the nAg₂O/AC Fenton system exhibited a dramatically enhanced fluorescence signal (435.3) of HTA, which was 6.2 times that of H₂O₂ system (70.5) and 2.9 times that of traditional Fe²⁺ Fenton system (149.6) reported by.⁴² To calculate decomposition efficiency of H₂O₂ in different system, total amount of H₂O₂ (represented by total amount of generated HTA) was received by fully heating 100 mL solution containing 32 μL H₂O₂ and 60 mg TA for 60 min. The results demonstrated that the decomposition efficiency in nAg₂O/AC Fenton system arrived at 73.7% (435.3/590.8 × 100%), which was significantly higher than 11.9% (70.5/590.8 × 100%) in H₂O₂ system and 25.3% (149.6/584.8 × 100%) in traditional Fe²⁺ Fenton system. Hence, it concluded that the nAg₂O/AC could remarkably facilitate the decomposition efficiency of H₂O₂ to produce more OH˙ in heterogeneous Fenton reaction and OH˙ is the major ROS accounting for removing bacteria and TC.

Based on the above discussion, a possible reaction mechanism of the nAg₂O/AC Fenton system was proposed as shown in Fig. 8. Firstly, reductive functional groups such as hydroxyl and aldehyde of AC reduced surface-bound Ag(I) in nAg₂O/AC to surface-bound Ag by mediated electron transfer process. Then, the generated Ag catalyzed H₂O₂ to produce a great quantity of OH˙ radicals. As a strong oxidizing species, the OH˙ radicals can effectively degrade micropollutants and inactivate bacteria based on oxidation by attacking cell membranes and proteins, enzymes and genes.⁴³ Meanwhile, the generated Ag(I) was transformed into Ag again since Ag(I)/Ag cycle was accelerated by the mediated electron transfer of AC. Therefore, the nAg₂O/AC could continuously catalyze H₂O₂ to generate OH˙ radicals for enhanced removal of bacteria and micropollutants.

Fig. 8 The proposed reaction mechanism of the nAg₂O/AC Fenton system.

4 Conclusions

In summary, the nAg₂O/AC composite was synthesized as a novel heterogeneous Fenton catalyst and achieved high removal efficiency for E. coli and TC in water via H₂O₂ activation. Mechanism investigations revealed that reductive functional groups of AC accelerated the Ag(I)/Ag cycle through mediated electron transfer and more Ag catalyzed H₂O₂ to generate abundant OH˙ radicals. The OH˙ radicals were confirmed to be the major role leading to bacterial inactivation and TC degradation by scavenging tests and ESR analysis. Besides, the effective decomposition rate of H₂O₂ was further quantitatively analyzed by fluorescence spectroscopy. The high removal efficiency at about neutral condition (pH 5–8) in the heterogeneous Fenton reaction made nAg₂O/AC more advantageous in actual water treatment. In addition, the nAg₂O/AC catalyst has good stability and reusability in 5 cycles and regeneration tests, indicating its considerable application prospect. These results demonstrated that nAg₂O/AC as a promising heterogeneous Fenton catalyst is expected to solve the combined contamination problem of bacteria and micropollutants in water. In fact, this work also provides a new perspective for the application of metal oxide as a catalyst for hydrogen peroxide or persulfate in water treatment.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Jianping Deng: experiment, methodology, software, writing – original draft preparation. Yong Liu: methodology, validation. Shuanglin Gui: validation, project administration. Qizhen Yi: data curation, project administration. Hanbing nie: writing – reviewing and editing, supervision.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors thank the National Natural Science Foundation of China (41977114, 41807338, 52060008).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra04604h

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