Degradation of ribavirin by Fe²⁺/PS oxidation technology: performance, mechanism and toxicity control

2.2.1 RBV degradation experiments

The degradation experiments of RBV were performed in 500 mL conical flasks. RBV solution with an initial concentration of 2.0 mg/L was prepared prior to the degradation reactions and added to the conical flask, following with the addition of hydrochloric acid or sodium hydroxide to adjust pH to the desired value. The reactions were initiated by adding a certain quality of PS powder according to the experimental conditions. The reactions system was stirred at 350 r/min. 10 mL of treated samples were taken at different times and filtered through a 0.22 μm polyethersulfone membrane for further analysis. All the experiments were conducted at least twice.

2.2.2 Qualitative determination of active components

Some studies have confirmed that the active substances present in the Fe²⁺/PS system are mostly SO₄^-· and ·OH; while some scholars have also proposed that FeO²⁺ is one of the active substances that sulfate being activated by ferrous (Wang et al., 2018). Therefore, in order to identify the active substances present in Fe²⁺/PS oxidation system, this study has set up the electron paramagnetic resonance spectroscopy (EPR) tests, the free radical quenching tests, and FeO²⁺ capturing tests.

The EPR tests were applied for directly identifying the active components produced in Fe²⁺/PS oxidation system. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) and 2,2,6,6-Tetramethylpiperidine (TEMP) were used to capture the active substances, such as SO₄^•-, •OH and ¹O₂, so as to form stable products DMPO-SO₄^-, DMPO-OH and TEMP-¹O₂, which all with the specific signal peaks (Lian et al., 2019).

Methanol (MeOH) had a large reaction rate constant with SO₄^-·（k_{SO4•−, MeOH} = 1.1 × 10⁷ M⁻¹S⁻¹）and ·OH (k_{•OH, MeOH} = 9.7 × 10⁸ M⁻¹S⁻¹), while tert butyl alcohol (TBA) had much larger reaction rate constant with ·OH (k_{•OH, TBA} = 6.0 × 10⁸ M⁻¹S⁻¹) than that with SO₄^-· (k_{SO4•−, TBA} = 4.0 × 10⁵ M⁻¹S⁻¹) (Liu et al., 2021). Thus, MeOH was selected as the quencher of SO₄^-· and ·OH, while TBA as the quencher of ·OH. The concentrations of MeOH and TBA were both 2.0 M, which 10⁵ times than that of RBV and could fully quench the produced free radicals during the reactions. The free radical quenching tests were carried out by adding a certain volume of MeOH and TBA in advance. The degradation rates of RBV were compared with the experiments without the quenching agents in order to assess the dominant species of free radicals.

2.2.3 Tracer experiment of sulfate radical and hydroxyl radical

The HO· can degrade nitrobenzene (NB) and atrazine (ATZ), whereas the SO₄^-· can only degrade ATZ. Therefore, by adding small amounts of NB and ATZ to the degradation system, the exposure values of SO₄^-· and HO· can be calculated based on the degradation rate and the second-order reaction constants with the active radicals. The specific formula for this calculation is as follows:

In Fe²⁺/PS oxidation system, there also existed the non-free radical active component FeO²⁺, which could react with tetramethylene sulfoxide (TMSO) to form tetramethylsulfoxide (TMSO₂). Thus, the FeO²⁺ capturing tests were carried out by adding 0.5 mM TMSO before the start of RBV degradation and detecting the content of TMSO₂ to calculate the concentration of FeO²⁺. The quantification of TMSO and TMSO₂ is achieved through High-performance liquid chromatography (HPLC). Chromatographic conditions include operating at 35 °C using a C18 column (4.6 × 150 mm, 5 µm), with an isocratic elution of acetonitrile/water (60/40 v/v) at a flow rate of 1 mL/min. The injection volume is set at 100 µL, and ultraviolet detection is conducted at a wavelength of 212 nm.

3.2.1 Identification of active components during RBV degradation

To directly confirm the presence of ·OH and SO₄^-· radicals during the oxidation of RBV in the Fe²⁺/PS system, EPR spectroscopy was employed. DMPO was used to capture free radicals, with the EPR spectra scanned using an electron spin resonance spectrometer. The EPR spectra, as shown in Fig. 2(a), revealed the detection of four DMPO-OH characteristic peaks (intensity ratio 1:2:2:1) and six DMPO-SO₄ peaks in the Fe²⁺/PS added experimental group. This confirmed the existence of ·OH and SO₄^-· during the RBV oxidation process in the Fe²⁺/PS system. The high degradation rate of RBV by Fe²⁺/PS oxidation further substantiates the role of ·OH and SO₄^-· in the system's degradation mechanism of RBV.

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Fig. 2

EPR spectra of (a) DMPO-OH and DMPO-SO₄, (b) TEMP-¹O₂ in RBV Fe²⁺/PS oxidation process and (c) TMSO₂ concentration for FeO²⁺ measure, for FeO²⁺ measure test, the [RBV]₀ = 2.0 mg/L, initial pH = 3,[PS]₀ = 4.0 mM, [PS/Fe²⁺] = 2/1, [CA]₀ = 0.5 mM, add in Fe²⁺ in twice, [TMSO]₀ = 0.5 mM).

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Additionally, the spectra revealed that in samples where only PS was added, neither DMPO-OH nor DMPO-SO₄ peaks were detected. However, a minor degradation of RBV was still observed in these PS-only experiments, suggesting the presence of active species other than ·OH and SO₄^-· in the Fe²⁺/PS system. EPR spectroscopy using TEMP as a radical trap was thus employed to detect these species. The EPR spectra, as shown in Fig. 2(b), indicated the presence of three characteristic TEMP-¹O₂ peaks with an intensity ratio of 1:1:1 in both PS-only and Fe²⁺/PS-added samples, signifying ¹O₂ as another active species involved in RBV oxidation. Notably, the intensity of TEMP-¹O₂ peaks in the Fe²⁺/PS group was significantly higher than in the PS-only group, indicating enhanced ¹O₂ generation upon the addition of Fe²⁺, ·OH and other reaction intermediates may ultimately be converted into ¹O₂ through a series of complex reaction pathways. Considering that no DMPO-OH was detected in the sample with only PS added, so the main source of ¹O₂ in the PS system may be the conversion of other intermediate products. Existing studies suggest that ¹O₂ can selectively oxidize and degrade recalcitrant pollutants in water, though its activity in degrading saturated alcohols like ethanol, methanol, and tert-butanol is negligible (Hong et al., 2021).

FeO²⁺ is another non-radical active component in the Fe²⁺/PS system (Zhou et al., 2023). Experiments under optimal conditions were initiated with the addition of 0.5 mM TMSO. Sampling at different intervals to detect TMSO₂ concentrations allowed for estimation of FeO²⁺ concentration changes, as illustrated in Fig. 2(c). The generation of TMSO₂ indicates the presence of FeO²⁺ in the Fe²⁺/PS system. However, the sulfonation rate of approximately 6.00 % indirectly suggests a minor contribution of FeO²⁺ to RBV degradation. This could be attributed to the instability of FeO²⁺ under acidic conditions, as previous study indicates a higher stability of FeO²⁺ in alkaline conditions, providing sufficient time for interaction with pollutants (Koppenol, 2022).

3.2.2 Contribution of active components in RBV degradation

Alcohol inhibition experiments were conducted to study the contributions of ·OH and SO₄^- to the degradation of RBV. The impact of various alcohols on RBV degradation is depicted in Fig. 3(a). It was observed that MeOH and TBA significantly inhibited RBV degradation, with MeOH exhibiting a higher inhibition rate than TBA. The degradation rates of RBV in the absence of alcohol, with MeOH, and with TBA were 96.01 %, 11.14 %, and 66.27 %, respectively, indicating the predominant contribution of SO₄^-, followed by ·OH. The addition of MeOH did not inhibit the action of ¹O₂ due to its low reaction rate with ¹O₂, suggesting that ¹O₂ does not play a major role in RBV degradation.

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Fig. 3

(a) Effect of different alcohols on the degradation rate of RBV ([RBV]₀ = 2.0 mg/L, initial pH = 3,[PS]₀ = 4.0 mM, [PS/Fe²⁺] = 2/1, [CA]₀ = 0.5 mM, add in Fe²⁺ in twice, [MeOH] = 2.0 M, [TBA] = 2.0 M), (b) Radical tracer experiments of SO₄^-· and OH·.

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Radical tracer experiments, as shown in Fig. 3(b), further indirectly confirm that ·OH and SO₄^- are the active components in the Fe²⁺/PS oxidation process of RBV, with the exposure value of SO₄^- being significantly higher than that of ·OH, indicating a stronger degradation effect of SO₄^- on RBV. At 10 mins into the reaction, there was a substantial increase in the exposure values of the radicals, due to the activation of PS by the second addition of Fe²⁺, generating a large amount of ·OH and SO₄^-.

3.2.3 RBV degradation products and pathways

Using HPLC-MS, ten products with m/z values of 113, 147, 198, 211, 241, 256, 283, 338, 340, and 344 were detected. Liu et al. (2021) identified five DPs of RBV with m/z values of 241, 211, 198, and 147. Thus, four identifiable DPs were determined as DP-1, DP-2, DP-3, and DP-4, with corresponding mass spectral parameters listed in Table 1. DP-1, with an m/z of 241, slightly lower than RBV's 245, indicates the presence of alcohol, carbonyl, and amide groups in RBV. The increased unsaturation suggests DP-1 as a hydroxyl dehydrogenation product. Researchers have demonstrated through quantum chemical calculations that the amide group is likely susceptible to attack (Liu et al., 2021). DP-2 and DP-3, based on their mass difference from DP-1, are hypothesized to be the de-alcoholized and de-amidized degradation products of DP-1, respectively. DP-3 may undergo C-N bond breakage and hydroxylation, transforming into DP-4. Therefore, a possible RBV degradation pathway, based on the above analysis, is proposed in Fig. 4. The mineralization of RBV in the system was determined to be 13.1 % by total organic carbon (TOC) analyzer(Fig. S5). Combined with the fact that the detected products of RBV are macromolecules it can be seen that the degradation of RBV by the PS/Fe²⁺ system is not sufficient.

Table 1 Mass spectrometry parameters of the four degradation products.

	Retention time	Molecular formula	Relative molecular mass	ESI(+)/MS (m/z)	Structural
DP-1	7.45 min	C8H8N4O5	240	241
DP-2	6.22 min	C7H6N4O4	210	211
DP-3	5.92 min	C7H7N3O4	197	198
DP-4	8.20 min	C5H6O5	146	147

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Fig. 4

Possible RBV degradation pathway.

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3.3.1 Impact of initial pH on RBV degradation

The effects of different initial pH levels on RBV degradation are presented in Fig. 5. Results indicate that acidic conditions generally promote higher degradation rates compared to neutral and alkaline conditions. At pH 9, the degradation rate drops to its lowest at 24.22 %. However, the optimal initial pH for RBV degradation is identified as 3, achieving a degradation rate of 82.18 %. The enhanced efficiency under acidic conditions is attributed to multiple factors: (1) Formation of Fe²⁺ complexes at pH > 4.0 impedes the reaction between Fe²⁺ and PS, leading to Fe²⁺ precipitation as Fe³⁺ ions, reducing the available Fe²⁺ and consequently diminishing PS activation (Xu and Li, 2010); (2) SO₄^-· exhibits higher redox potential in acidic solutions, resulting in higher degradation rates (Romero et al., 2010). At pH 2, the degradation rate of RBV is 54.57 %, lower than at pH 3, possibly due to reduced radical quantities at lower pH levels, as corroborated by Khan and Adewuyi, (2010).

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Fig. 5

Effect of different initial pH on the degradation of RBV. Initial experimental conditions: [RBV]₀ = 8.0 μM, [PS]₀ = 4.0 mM, [Fe²⁺]₀ = 2.0 mM.

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3.3.2 Impact of reagents on RBV degradation

The influence of varying PS concentrations on RBV degradation is illustrated in Fig. 6(a). An initial increase in degradation rate is observed with rising PS molar concentrations, peaking when the concentration shifts from 2 mM to 4 mM, elevating RBV degradation from 71.70 % to 83.88 %. This enhancement is attributed to an increase in active radicals generated upon PS activation. However, degradation stabilizes with further PS concentration increments, as demonstrated at 6 mM and 8 mM, yielding RBV removal rates of 81.39 % and 80.69 %, respectively. This indicates that at 4 mM PS, sufficient active radicals for RBV degradation are produced under the experimental conditions of this study.

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Fig. 6

Effects of (a) different PS concentrations ([RBV]₀ = 2.0 mg/L, initial pH = 3, [Fe²⁺]₀ = 2.0 mM), (b) different PS/Fe²⁺ molar ratio ([RBV]₀ = 2.0 mg/L, initial pH = 3, [PS]₀ = 4.0 mM) and (c) different Fe²⁺ dosage on the degradation of RBV([RBV]₀ = 2.0 mg/L, initial pH = 3, [PS]₀ = 4.0 mM, [PS/Fe²⁺] = 2/1, [CA]₀ = 0.5 mM) on the degradation of RBV.

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Additionally, altering PS/Fe²⁺ molar ratios under a 4 mM PS concentration was explored for its effect on RBV degradation, as shown in Fig. 6(b). RBV removal rates initially increase then decrease with rising Fe²⁺ concentrations. A PS/Fe²⁺ molar ratio of 1/0.1 results in a 55.47 % degradation rate at reaction completion. Increasing Fe²⁺ to a 1/0.5 PS/Fe²⁺ ratio enhances the degradation rate to 83.86 %. However, further Fe²⁺ increments hinder RBV degradation, with a 1/1 PS/Fe²⁺ ratio resulting in only a 78.59 % degradation rate. Similar findings by Chen et al. (2009) suggest that organic matter degradation initially increases and then decreases with rising Fe²⁺ concentrations. They explain that an increase in Fe²⁺ enhances SO₄^-· generation from PS activation (Eq. (4), thus elevating RBV degradation rates. Conversely, excess Fe²⁺ competes for SO₄^-·, forming sulfate (Eq. (5), thereby diminishing RBV degradation capacity. Consequently, a PS/Fe²⁺ molar ratio of 1/0.5 is identified as the optimal ratio for RBV degradation in the Fe²⁺/PS system.

The impact of different Fe²⁺ addition methods on RBV degradation was additionally investigated, as shown in Fig. 6(c). In the Fe²⁺/PS system, a single addition of Fe²⁺ resulted in a lower RBV degradation rate compared to batch additions. The RBV removal rate with a single Fe²⁺ addition was 94.27 %, whereas batch additions of Fe²⁺ consistently achieved degradation rates over 98.00 %. The highest removal rate, reaching 98.70 %, was observed with two separate additions of Fe²⁺. Shang et al. (2019) also discovered that batch addition of Fe²⁺ increased the removal rate of diatrizoate by 61 %. This phenomenon can be attributed to the gradual, batch addition of Fe²⁺, leading to a slower generation of SO₄^-· in the system, which allows for more effective interaction between RBV and the radicals, thereby enhancing the degradation rate.

3.3.3 Influence of reaction environment on RBV degradation

In addition to initial pH and reagent addition, other coexisting components in the reaction environment can also impact the degradation of RBV by the Fe²⁺/PS oxidation system. Citric acid (CA), a green chelating agent, has been confirmed by Han et al. (2015) to significantly enhance the activation of Fe²⁺ on PS. Consequently, this study explored the impact of different concentrations of CA on RBV degradation in the Fe²⁺/PS system, as shown in Fig. 7(a). When the CA concentration increased from 0 mM to 0.5 mM, the removal rate of RBV rose from 83.02 % to 95.53 %. However, further increases in CA concentration led to a decrease in RBV removal. At a CA concentration of 2 mM, the RBV removal rate was 87.27 %. Zeng et al. (2021) reached similar conclusions in his study on the degradation of naphthalene solutions using Fe²⁺ activated persulfate coupled with CA, attributing the effect to CA preventing Fe²⁺ precipitation and promoting the generation of active radicals. Excessive CA forms highly stable chelates with Fe²⁺, which is not conducive to catalytic reaction of Fe²⁺ (Rastogi et al., 2009). Huang et al. (2005)also mentioned in his research that high concentrations of citrate compete with target pollutants for SO₄^-·.

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Fig. 7

Effects of (a) different molar ratios of citric acid (CA) ([RBV]₀ = 2.0 mg/L, initial pH = 3, [PS]₀ = 4.0 mM, [PS/Fe²⁺] = 2/1) and (b) different halogen ion concentrations on the degradation of RBV ([RBV]₀ = 2.0 mg/L, initial pH = 3, [PS]₀ = 4.0 mM, [PS/Fe²⁺] = 2/1, [CA]₀ = 0.5 mM, add in Fe²⁺ in twice).

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Halide ions, commonly found in natural water bodies, were detected in a river in Shenzhen, China, with concentrations of Cl^-, Br^-, and I^-, where Cl^- reached up to 3200 mg/L (Shi et al., 2018). Yang et al. (2014) noted that halides compete with pollutants for reactive species, making it essential to understand their influence on RBV degradation in the Fe²⁺/PS system. The effect of varying concentrations of halide ions on RBV degradation in the Fe²⁺/PS process is shown in Fig. 7(b). All three halides inhibited RBV degradation, with the effect intensifying at higher ion concentrations. This correlates with halide reactions forming weaker halogen radicals compared to ·OH and SO₄^-·, as indicated in Eqs. (6) – (10) (Zhang and Parker, 2018). For instance, Cl· and Cl₂^-· have lower oxidation potentials than SO₄^-· and ·OH. Thus, the presence of halide ions reduces the Fe²⁺/PS removal rate of RBV. A first-order kinetics fit was conducted to understand the rate changes under different conditions.

3.3.4 RBV degradation kinetics

Pseudo first-order kinetic fitting was applied to RBV degradation under various operational conditions (Table 2) as Eq. (11).

It was found that during the initial 5 min, the degradation process followed a first-order kinetic model, with fitting R² values above 0.969. However, as the reaction progressed, the process no longer followed this model, with R² values dropping to 0.491 – 0.758 when fitting 0 – 60 min data. In the subsequent degradation process, the degradation rate decreases, resulting in no longer following the first-order reaction kinetics. This could be attributed to the rapid consumption of radicals due to RBV degradation, resulting in a deviation from first-order kinetics as reactant concentrations decreased. The result indicates the relatively rapid degradation efficiency of RBV by PS/Fe²⁺. Besides, it also suggests that achieving complete degradation of RBV requires a more sufficient dosage of reagents.

Experiments on factors affecting the degradation efficiency in the Fe²⁺/PS system indicated that optimal RBV degradation was achieved at pH = 3, [PS] = 4 mM, and a PS/Fe²⁺ molar ratio of 1/0.5. Kinetic study demonstrated a correlation between apparent kinetic constants and reaction conditions, with the highest constants observed under optimal degradation conditions.