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In this study, a potassium ferrate (K2FeO4)-modified biochar (Fe–BC) was prepared and characterized. Afterwards, Fe–BC was applied to activated periodate (PI) to degrade tetracycline (TC), an antibiotic widely used in animal farming. The degradation effects of different systems on TC were compared and the influencing factors were investigated. In addition, several reactive oxygen species (ROS) generated by the Fe–BC/PI system were identified, and TC degradation pathways were analyzed. Moreover, the reuse performance of Fe–BC was evaluated. The results exhibited that the Fe–BC/PI system could remove almost 100% of TC under optimal conditions of [BC] = 1.09 g/L, initial [PI] = 3.29 g/L, and initial [TC] = 20.3 mg/L. Cl−, HCO3−, NO3−, and humic acid inhibited TC degradation to varying degrees in the Fe–BC/PI system due to their quenching effects on ROS. TC was degraded into intermediates and even water and carbon dioxide by the synergistic effect of ROS generated and Fe on the BC surface. Fe–BC was reused four times, and the removal rate of TC was still maintained above 80%, indicating the stable nature of Fe–BC.

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  • Fe–BC with excellent morphology and composition was successfully prepared.

  • Fe–BC could effectively activate periodate in degrading tetracycline (TC).

  • The response surface methodology was utilized to determine the optimum conditions.

  • The reactive oxygen species generated were responsible for TC degradation.

advanced oxidation process, modified biochar, periodate, potassium pertechnetate, tetracycline
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Nowadays, due to the continuous pollution of the water environment, more and more emerging pollutants with negligible concentrations (ng/L–μg/L), such as pesticides, pharmaceuticals, detergents, and flame retardants (Teodosiu et al. 2018), have appeared in both surface water and groundwater. Emerging pollutants are not easily degraded under natural conditions; thus, treating these pollutants at the end of the sewage plant or before the water supply plant will be conducive to the sustainable use of water resources. Conventional methods such as coagulation, filtration, adsorption, and biological oxidation are ineffective in removing stubborn emerging pollutants (Bracamontes-Ruelas et al. 2022). The advanced oxidation processes (AOPs) based on several oxidizing agents (peroxymonosulfate (PMS), peroxydisulfate (PDS), peroxyacetic acid (PAA), and periodate (PI)) have received a lot of attention because of their strong degradation ability for stubborn organic pollutants. These oxidizers can produce some reactive oxygen species (ROS, ·OH, ⁠, ⁠, ⁠, ⁠, ⁠, ⁠) in situ to destroy stubborn organic pollutants as activated by certain means, including ultraviolet (UV), transition metals (Fe, Co, Ni, and Cu), and carbon materials (activated carbon and biochar (BC)). For instance, UV/PMS (Sharma et al. 2015), UV/PDS (Nihemaiti et al. 2018), UV/PAA (Zhang & Huang 2020), UV/PI (Bendjama et al. 2018), Fe/PMS (Li et al. 2021), Co/PDS (Li et al. 2022), Cu/PAA (Wu et al. 2024), Fe–Ni(or Cu)/PI (Lee et al. 2014), activated carbon/PMS (Yang et al. 2015), BC/PDS (Miao et al. 2022), BC/PAA (Zhang et al. 2024), and activated carbon/PI (Li et al. 2016) were applied to treat some organic pollutants (bisphenol A, 4-chlorophenol, phenol, Rhodamine B, chlorazol black, tetracycline (TC), sulfamethoxazole, acid orange 7, and carbamazepine) in water and obtained effective degradation rates.

Compared with the above-mentioned oxidizing agents (PMS, PDS, and PAA), PI is chemically more stable, and there are no residual sulfate ions and less sludge in the reaction process. Moreover, the bond energy of PI is small and easy to activate, and PI can be used to not only oxidize organic matter but also inactivate microorganisms (Sukhatskiy et al. 2023). Among the several methods of activating PI, UV can activate PI to generate , , and O(3P) (Song et al. 2024), which can oxidize organic matter or sterilize it. However, the UV activation consumes energy and is affected by the transmittance of the solution. Transition metal activation can ensure high organic removal efficiency (Du et al. 2019); nevertheless, the residual transition metals in the water are prone to cause secondary water pollution (He et al. 2022a, 2022b). In comparison, the activation of PI using BC has excellent effectiveness in removing organic pollutants in water (Oliveira et al. 2017). BC is derived from the pyrolysis of carbon-containing biomass, especially waste biomass, whose reuse has an environmental protection effect (Panahi et al. 2020). BC surface contains several functional groups that can activate PI to generate free or non-free radicals to degrade organic matter (Dai et al. 2023). However, unmodified BC is not rich in functional groups and its ability to activate PI is limited (Song et al. 2023).

The modification of BC via transition metals can lead to the formation of more functional groups on the surface of BC (Wang & Wang 2019). Moreover, the transition metals are immobilized on the surface or in the pores of the BC, which can reduce their leakage into the water, and the transition metals can also synergize with the BC to activate the PI (He et al. 2021). Zhuo et al. (2024) employed a Fe–BC/PI system to remove acetaminophen (ACT, 1 mg/L), resulting in the complete degradation of ACT within 1 min at pH = 11. The main active species generated in this process were ·OH, 1O2, Fe (IV), and , which were responsible for the degradation of ACT. Gong et al. (2024) synthesized an Mn–BC composite and investigated its capacity to degrade methylene blue (MB) in the presence of PI. The results exhibited that 97.4% of MB was removed within 30 min under the synergistic effect of ⁠, 1O2, and ·OH generated in the Mn–BC/PI system. There are also several transition metal-modified BCs such as magnetic Fe–BC (Xiang et al. 2023), Fe/Cu–BC (He et al. 2021), Fe/Mn–BC (He et al. 2022a, 2022b), and MnxOy–BC (Fang et al. 2022), which were used to activate the PI to degrade diphenylamine dithiophosphoric acid, diclofenac sodium, thiacloprid, and oxytetracycline, respectively. Fe, as a transition metal, is cheap and non-toxic; therefore, PI activated via Fe in removing organics is very popular (Su et al. 2023).

In this study, a novel Fe–BC was prepared and characterized. The as-prepared Fe–BC was applied to activate PI to decontaminate TC, a typical antibiotic medicine used in livestock and aquaculture. The effect of the Fe–BC/PI system on TC removal was examined, some influencing factors were optimized via response surface methodology (RSM), and the production of active species and TC degradation pathways were analyzed. This study is expected to provide a reference for researchers studying the degradation of emerging pollutants by the modified BC/PI system.

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Chemical reagents

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The chemical reagents involved in this study were purchased from two companies. Sodium periodate (NaIO4), potassium pertechnetate (K2FeO4), sodium chloride (NaCl), sodium bicarbonate (NaHCO3), and sodium nitrate (NaNO3) were purchased from Sinopharm Chemical Reagent Co. TC (C22H24N2O8), humic acid (HA, C9H8Na2O4), tert-butanol (C4H10O), para-benzoquinone (C6H4O2), and furfuryl alcohol (C5H6O2) were purchased from Shanghai McLean Biochemical Technology Co. All these reagents were analytically pure. The water used to prepare the aqueous TC solution was deionized (DI) water.

Preparation and characterization of BC

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Primary BC was first prepared. The lees were cleaned with DI water, ground into powder, screened for 80 mesh, and placed in a tube furnace for pyrolysis. The pyrolysis conditions were as follows: nitrogen flow rate of 100 mL/min, heating rate of 10 °C/min, and end temperature of 800 °C (maintained for 2 h). Primary BC was obtained at the end of the pyrolysis process. In the preparation of modified BC, the lees powder (80 mesh, 10 g) was first mixed and stirred with potassium pertechnetate (4 g) and then pyrolyzed in a tube furnace. The pyrolysis conditions were the same as those of the primary BC. The modified BC obtained was denoted as Fe–BC. The prepared materials were characterized by the scanning electron microscope (SEM, ZEISS MERLIN Compact, Germany), the energy dispersive spectrometer (EDS, Thermo Scientific K-Alpha + , USA), and X-ray diffraction (XRD, Rigaku Ultima IV, Japan) to explore their morphology, elemental composition, and compound forms.

Experimental methods

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The reactor used in the experiment was a 250-mL beaker. The beaker was filled with 100 mL of aqueous TC solution (20 mg/L), BC (or Fe–BC, 0.1 g), PI (0.33 g), and BC (or Fe–BC, 0.1 g) + PI (0.33 g) according to the designed program, and the solution was stirred with a magnetic stirrer to trigger a reaction. The sampling times were 10, 30, 60, 90, 120, and 150 min. The residual TC concentration was measured at 370 nm via a UV–Vis spectrophotometer (TU-1810, Beijing Pure Analytics General Co.).

The effects of pH, temperature, Cl−, ⁠, ⁠, and HA on TC degradation were investigated with the following parameter ranges: pH (3, 6, and 9), temperature (15, 25, and 35°C), anions Cl−, ⁠, (10 and 20 mg/L), and HA (5 and 10 mg/L).

After the experiment was completed, the spent Fe–BC in solution was extracted by vacuum filtration and washed repeatedly with DI water, dried, and used to repeat the experiment to examine the stability of the Fe–BC in the PI-AOP system.

Quenching experiments were carried out to investigate the ROS generated in the Fe–BC/PI system. Quenching agents such as tert-butanol (TBA), furfuryl alcohol (FFA), and p-benzoquinone (p-BQ) were chosen to quench ·OH, 1O2, and  based on differences in the corresponding reaction rate constants. The parameters of the reaction were the Fe–BC dosage of 1.09 g/L, the PI concentration of 3.29 g/L, the TC concentration of 20.3 mg/L, and the temperature of 25 °C. The molar ratio of TBA:PI was 1000:1, the FFA concentration was 0.2 M, and the p-BQ concentration was 5 mM.

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Characterization results

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SEM images of pristine BC and Fe–BC are shown in Figure 1(a) and  1(b), respectively. The pristine BC possessed a porous structure with a relatively smooth surface, while the modified BC had many rough particles on the porous structure. In the process of lees pyrolysis, the organic components were cracked and volatilized, resulting in a porous structure of BC or Fe–BC. Moreover, the particles on the surface of modified BC may be iron oxides. According to the EDS spectrum of Fe–BC in Figure 1(c), the modified BC was mainly composed of elements such as Fe, C, and O, confirming the successful doping of Fe into the BC. In terms of XRD analysis in Figure 1(d), BC showed a wide diffraction peak at 22° and 40°, which suggested graphite carbon of amorphous structure induced by pyrolysis (Xiang et al. 2023). It could be seen that diffraction peaks of Fe2O3, FeSiO3, Fe, and Fe3O4 appeared near 2θ = 30.3°, 32.9°, 44.6°, and 65.0° (Dong et al. 2018; Xiang et al. 2023), respectively. This indicated that Fe monomers and iron oxides with different valence states were generated in the BC through the modification of K2FeO4, which were successfully embedded in the BC. Moreover, the dimensions of the BC and Fe–BC were determined. BC had a specific surface area of 260.4 m2/g and an average pore size of 2.38 nm. The specific surface area (264.58 m2/g) and pore size (2.68 nm) of Fe–BC did not differ much from that of BC. Surprisingly, Si was detected in BC, which we hypothesized to be derived from the lees raw material. The raw material for the preparation of BC was made from wine lees, which is a kind of waste from brewing products. Therefore, Si in BC may be derived from the lees feedstock. In addition, it was found that the original XRD diffraction peak of BC disappeared after modification. This may have something to do with the sampling point of the XRD assay. During diffraction of X-ray, substances such as Fe cover the carbon, resulting in the modified BC peaks not being recognized.
Figure 1
SEM image of (a) BC and (b) Fe–BC; (c) EDS spectrum of Fe–BC; and (d) XRD patterns of Fe–BC and BC.
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SEM image of (a) BC and (b) Fe–BC; (c) EDS spectrum of Fe–BC; and (d) XRD patterns of Fe–BC and BC.

Figure 1
SEM image of (a) BC and (b) Fe–BC; (c) EDS spectrum of Fe–BC; and (d) XRD patterns of Fe–BC and BC.
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SEM image of (a) BC and (b) Fe–BC; (c) EDS spectrum of Fe–BC; and (d) XRD patterns of Fe–BC and BC.

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Comparison of different conditions for TC degradation

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Figure 2 demonstrates the degradation effect of TC under different conditions (only BC, only PI, only Fe–BC, BC + PI, and Fe–BC + PI). The results showed that the degradation rates of TC by the different systems were in descending order: Fe–BC + PI (99.2%), BC + PI (82.6%), PI (51.6%), Fe–BC (42.1%), and BC (32.3%). This suggested that the presence of PI improved the removal of TC by BC or Fe–BC; i.e. both BC and Fe–BC could effectively activate PI to degrade TC. However, Fe–BC possessed a better PI activation effect than that of BC. Because BC activated PI by the actions of surface functional groups and electron transfer, nevertheless, Fe–BC activated PI by redox action besides the previous actions. Therefore, Fe–BC could activate PI in more ways and degrade TC effectively. Previous studies reported that Fe and BC were capable of activating PI to degrade organic matter in either a radical pathway (Xiang et al. 2023) or a non-radical pathway (He et al. 2023), and they exerted a synergistic degradation effect (Zhuo et al. 2024), which was also accompanied by electron transfer (He et al. 2022a, 2022b). PI alone was also capable of degrading 51.6% of TC due to its redox potential of 1.6 V (Zou et al. 2024), but the degradation efficiency was half that of the Fe–BC/PI system. It was also clear that the removal of TC by Fe–BC and BC was mainly based on the porous nature of the materials leading to adsorption.
Figure 2
TC removal under different systems. Conditions: [TC] = 20 mg/L, [BC] = [Fe–BC] = 1 g/L, [PI] = 3.3 g/L, T = 25°C, and pH = 3.
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TC removal under different systems. Conditions: [TC] = 20 mg/L, [BC] = [Fe–BC] = 1 g/L, [PI] = 3.3 g/L, T = 25°C, and pH = 3.

Figure 2
TC removal under different systems. Conditions: [TC] = 20 mg/L, [BC] = [Fe–BC] = 1 g/L, [PI] = 3.3 g/L, T = 25°C, and pH = 3.
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TC removal under different systems. Conditions: [TC] = 20 mg/L, [BC] = [Fe–BC] = 1 g/L, [PI] = 3.3 g/L, T = 25°C, and pH = 3.

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Several factors optimized via the RSM

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With the help of Design Expert 10 software, RSM analysis was performed based on the Box–Behnken design with the initial PI concentration, initial Fe–BC dosage, and initial TC concentration as the independent variables and the TC removal as the response value. The initial pH was 6, the reaction time was 150 min, and the reaction temperature was 25 °C in each experimental protocol. The levels and coding of the influencing factors are shown in Table 1. The specific protocols and the corresponding TC removals are exhibited in Table 2. Through software analysis, the relationship between the variables and the response values conformed to the quadratic polynomial regression equation (Equation (1)). Table 3 presents the analysis of variance (ANOVA) for TC removal. In Table 3, the corresponding p-value under the model hierarchy was <0.0001, and the F-value was 39.96. For the RSM model, a larger F-value and a smaller p-value mean a more realistic model fit. If the p-value was less than 0.05, the model was considered significant (Ntambwe Kambuyi et al. 2019). The model of this experiment was more significant with a p-value of <0.0001. The F-value of the misfit level was 0.3580, which indicated that the misfit term was not significant, indicating a high level of modeling accuracy. Furthermore, the R2 of the model was 0.9809 and CV% was 0.9743%. Both values similarly illustrated the applicability of the model.

Table 1

Factor levels of experiment design

FactorsVariablesLevels
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PI concentration (g/L) A 1.1 2.2 3.3 
Fe–BC dosage (g/L) B 0.5 1.0 1.5 
TC concentration (g/L) C 0.02 0.03 0.04 
FactorsVariablesLevels
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PI concentration (g/L) A 1.1 2.2 3.3 
Fe–BC dosage (g/L) B 0.5 1.0 1.5 
TC concentration (g/L) C 0.02 0.03 0.04 
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Table 2

Operating parameters and removal rates of TC

RunABCResponse: TC removal (%)
1 −1 1 0 81.1 
2 −1 0 1 78.2 
3 0 0 0 82.3 
4 −1 0 −1 83.6 
5 1 1 0 90.6 
6 0 1 1 83.0 
7 1 0 −1 92.3 
8 0 0 0 83.8 
9 0 0 0 84.8 
10 1 −1 0 87.1 
11 0 1 −1 90.3 
12 0 −1 −1 88.0 
13 −1 −1 0 79.1 
14 0 −1 1 81.6 
15 0 0 0 84.4 
16 0 0 0 83.4 
17 1 0 1 85.1 
RunABCResponse: TC removal (%)
1 −1 1 0 81.1 
2 −1 0 1 78.2 
3 0 0 0 82.3 
4 −1 0 −1 83.6 
5 1 1 0 90.6 
6 0 1 1 83.0 
7 1 0 −1 92.3 
8 0 0 0 83.8 
9 0 0 0 84.8 
10 1 −1 0 87.1 
11 0 1 −1 90.3 
12 0 −1 −1 88.0 
13 −1 −1 0 79.1 
14 0 −1 1 81.6 
15 0 0 0 84.4 
16 0 0 0 83.4 
17 1 0 1 85.1 
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Table 3

ANOVA for TC removal

SourceSum of squaresdfMean squareF-valuep-Value
Model 244.54 9 27.17 39.96 <0.0001 Significant 
A 136.95 1 136.95 201.42 <0.0001  
B 10.58 1 10.58 15.56 0.0056  
C 86.46 1 86.46 127.16 <0.0001  
AB 0.5625 1 0.5625 0.8273 0.3933  
AC 0.8100 1 0.8100 1.19 0.3112  
BC 0.2025 1 0.2025 0.2978 0.6022  
A2 0.0380 1 0.0380 0.0559 0.8199  
B2 2.90 1 2.90 4.27 0.0777  
C2 5.62 1 5.62 8.26 0.0238  
Residual 4.76 7 0.6799    
Lack of fit 1.01 3 0.3358 0.3580 0.7874 Not significant 
Pure error 3.75 4 0.9380    
Cor total 249.30 16     
SourceSum of squaresdfMean squareF-valuep-Value
Model 244.54 9 27.17 39.96 <0.0001 Significant 
A 136.95 1 136.95 201.42 <0.0001  
B 10.58 1 10.58 15.56 0.0056  
C 86.46 1 86.46 127.16 <0.0001  
AB 0.5625 1 0.5625 0.8273 0.3933  
AC 0.8100 1 0.8100 1.19 0.3112  
BC 0.2025 1 0.2025 0.2978 0.6022  
A2 0.0380 1 0.0380 0.0559 0.8199  
B2 2.90 1 2.90 4.27 0.0777  
C2 5.62 1 5.62 8.26 0.0238  
Residual 4.76 7 0.6799    
Lack of fit 1.01 3 0.3358 0.3580 0.7874 Not significant 
Pure error 3.75 4 0.9380    
Cor total 249.30 16     
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Figure 3(a) illustrates the relationship between actual and predicted values. It was found that the data points were uniformly distributed around the diagonal line Y = X, indicating that the model could predict the TC removal well. Figure 3(b) gives the dual effect of changes in PI concentration and Fe–BC dosage on TC degradation. As can be seen from Figure 3(b), the shift from blue to orange-red color implied an increase in the TC removal rate. Namely, with the increase of PI concentration and Fe–BC dosing, the response volume TC removal rate was increasing. The effect of PI on TC removal was more significant. This was due to the fact that the increase in PI concentration resulted in the production of more free radicals during activation and they accelerated the TC degradation. The increase in Fe–BC dosing also provided more effective active sites, which improved TC removal. The interactive effects of PI and TC concentrations on TC degradation are shown in Figure 3(c). The interaction between PI and TC concentrations was not significant. The response values of TC removal increased with an increasing PI concentration and a decreasing initial TC concentration. The interaction effect of Fe–BC dosage and TC concentration on the TC degradation rate is shown in Figure 3(d). The shape of the contour lines in the figure was close to a circle, indicating that the interaction between the effect of Fe–BC dosing and initial TC concentration on the TC degradation rate was not significant. This indicated that the trends of the effects of changes in Fe–BC dosage and initial TC concentration on the TC degradation rate were relatively independent over the range considered.
Figure 3
(a) Predicted value and actual value; (b) the effect of Fe–BC dosage and PI concentration on TC removal; (c) the effect of TC concentration and PI concentration on TC removal; and (d) the effect of Fe–BC dosage and TC concentration on TC removal.
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(a) Predicted value and actual value; (b) the effect of Fe–BC dosage and PI concentration on TC removal; (c) the effect of TC concentration and PI concentration on TC removal; and (d) the effect of Fe–BC dosage and TC concentration on TC removal.

Figure 3
(a) Predicted value and actual value; (b) the effect of Fe–BC dosage and PI concentration on TC removal; (c) the effect of TC concentration and PI concentration on TC removal; and (d) the effect of Fe–BC dosage and TC concentration on TC removal.
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(a) Predicted value and actual value; (b) the effect of Fe–BC dosage and PI concentration on TC removal; (c) the effect of TC concentration and PI concentration on TC removal; and (d) the effect of Fe–BC dosage and TC concentration on TC removal.

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By utilizing this model, the optimal conditions for the TC removal were obtained as follows: BC dosage of 1.09 g/L, initial PI concentration of 3.29 g/L, and initial TC concentration of 20.3 mg/L. Under this optimal condition, the degradation rate of TC was expected to reach 92.8%. These parameters provide a reference for the experimental operation to optimize the degradation effect in real production or application.
(1)

Effect of other factors on TC degradation

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After the previous RSM optimization, the optimal PI concentration, BC dosage, and TC concentration were determined, and the effects of pH, temperature, various anions, and natural organic matter (represented by HA) on TC removal still need to be examined. Figure 4(a) shows the effect of pH on the TC removal in the Fe–BC/PI system. It can be seen that the best pH value was 3, at which the TC removal rate was 99% within 150 min. In contrast, there was almost no difference between the TC removal rates obtained at pH 6 and 9, which were around 90%. The redox potential of PI under acidic conditions is 1.6 V, while that of PI under alkaline conditions is only 0.7 V, which indicates that PI can perform better redox action at low pH (Koprivica et al. 2016). Moreover, at lower solution pH, the numerous H+ in solution may react with free electrons to form hydrogen radicals (H·), which can further react with PI to form more oxidizing-capable radicals (Li et al. 2016). However, the degradation efficiencies at pH 6 and 9 were also relatively high, suggesting that the Fe–BC/PI system was suitable for a wide range of pH, which would facilitate the application of this system. As the reviewer mentioned, pH does have some effect on the charged state of substances. It was found that pH had some effect on the charged state of substances. According to the previous study (Dantas et al. 2017), the neutral form (H2TC) will be predominant when 3.5 < pH < 6.5, while the anionic form (HTC−) will be predominant in the pH range of 8–9. In our reaction system, the effect of pH on TC degradation was not significant. The highest rate of TC degradation was observed at pH = 3. However, the degradation rate was comparable at pH = 6 and 9, and the decrease was not significant compared with that of pH = 3. The effect of temperature on the degradation TC of the Fe–BC/PI system is shown in Figure 4(b). The effects of three temperatures (15, 25, and 35°C) were investigated, and the results showed that there was little difference in the removal rate of TC in this temperature range. The reason may be that the temperature range of this experiment was low and did not have enough energy to activate the PI. In the study by Lu et al. (2024), the heat/PI process showed excellent efficiency in removing TC with temperature enhanced from 30 to 80 °C. Moreover, the rate of TC degradation increased more rapidly when the temperature was greater than 50 °C. The energy from high temperatures can break down PI into and O−·, which can oxidize TC. Figure 4(c)–(f) presents the effect of Cl−, ⁠, ⁠, and HA on TC degradation, respectively. It was found that 10 mg/L Cl− had no significant inhibitory effect on TC degradation. However, 20 mg/L of Cl− exerted slight inhibition on TC removal. Different from the effect of Cl−, the presence of (10 and 20 mg/L) reduced TC removal by 10% compared with the control level. However, the inhibitions of and HA were similar to that of Cl−, with slight inhibition at elevated concentrations. These anions, along with HA, are capable of reacting with the generated free radicals, thus causing a decrease in the oxidizing capacity of the system (Dai et al. 2023).
Figure 4
Some factors affecting TC degradation: (a) pH; (b) temperature; (c) Cl−; (d) ; (e) ; (f) HA. Conditions: [BC] = 1.09 g/L, [PI] = 3.29 g/L, [TC] = 20.3 mg/L, pH = 3.0, and T = 25 °C (unless otherwise specified).
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Some factors affecting TC degradation: (a) pH; (b) temperature; (c) Cl−; (d) ⁠; (e) ⁠; (f) HA. Conditions: [BC] = 1.09 g/L, [PI] = 3.29 g/L, [TC] = 20.3 mg/L, pH = 3.0, and T = 25 °C (unless otherwise specified).

Figure 4
Some factors affecting TC degradation: (a) pH; (b) temperature; (c) Cl−; (d) ; (e) ; (f) HA. Conditions: [BC] = 1.09 g/L, [PI] = 3.29 g/L, [TC] = 20.3 mg/L, pH = 3.0, and T = 25 °C (unless otherwise specified).
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Some factors affecting TC degradation: (a) pH; (b) temperature; (c) Cl−; (d) ⁠; (e) ⁠; (f) HA. Conditions: [BC] = 1.09 g/L, [PI] = 3.29 g/L, [TC] = 20.3 mg/L, pH = 3.0, and T = 25 °C (unless otherwise specified).

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Reuse performance of Fe–BC

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The removal rates of TC after four reuses of Fe–BC are given in Figure 5, i.e., the removal rates of TC degraded by four cycles were 94.1, 90.9, 86.2, and 85.2%, respectively. The decrease in degradation was attributed to the chemical changes caused by the adsorption of intermediate products on the Fe–BC surface and surface oxidation. The adsorption of intermediates may lead to the reduction of active sites and the specific surface area, thus hindering the activation of PI by Fe–BC. In addition, the chemical changes on the Fe–BC surface are also an important factor affecting the catalytic performance as it determines the active sites available for the PI. There is also the oxidation of functional groups and carbon structures on the surface of the Fe–BC during the catalytic process that may lead to a decrease in the removal performance. However, after four cycles, the Fe–BC could still maintain more than an 80% removal rate, which proved that the Fe–BC has good reusability.
Figure 5
BC reuse performance assessment. Conditions: [Fe–BC] = 1.09 g/L, [PI] = 3.29 g/L, [TC] = 20.3 mg/L, pH = 3.0, and T = 25 °C.
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BC reuse performance assessment. Conditions: [Fe–BC] = 1.09 g/L, [PI] = 3.29 g/L, [TC] = 20.3 mg/L, pH = 3.0, and T = 25 °C.

Figure 5
BC reuse performance assessment. Conditions: [Fe–BC] = 1.09 g/L, [PI] = 3.29 g/L, [TC] = 20.3 mg/L, pH = 3.0, and T = 25 °C.
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BC reuse performance assessment. Conditions: [Fe–BC] = 1.09 g/L, [PI] = 3.29 g/L, [TC] = 20.3 mg/L, pH = 3.0, and T = 25 °C.

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Recognition of ROS

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In order to recognize the ROS generated in the Fe–BC/PI system, TBA, FFA, and p-BQ were employed to quench ·OH, 1O2, and ⁠, respectively, with rate constants of k(TBA/·OH) = (3.8–7.6) × 108 M−1 s−1 (Gao et al. 2021), k(FFA/1O2) = 1.2 × 108 M−1s−1 (He & O'Shea 2020), k(p-BQ/·OH) = 1.2 × 109 M−1s−1 (Guo et al. 2021), and  (Guo et al. 2021). The quenching effects of TBA, FFA, and p-BQ are described in Figure 6, respectively. Under the controlled condition without quencher, the TC removal rate was 99.2% at 150 min, whereas it decreased to 90.5% after the addition of TBA alone, which indicated the generation of ·OH in the reaction. It was also seen that the reaction was not completely quenched and that most of the TC was still removed, confirming that other reactive species were generated in the reaction. When FFA alone was added to the Fe–BC/PI system, TC removal decreased from 99.2% at the control point to 75.6%, indicating that 1O2 was produced in the reaction. According to the k-value above, p-BQ can react with ·OH as well as . When p-BQ was introduced to the Fe–BC/PI system, the TC removal rate dropped dramatically (from 99.2 to 60.5%), which suggested that in addition to the production of ·OH, was also generated.
Figure 6
Inhibition by quenching agents. Conditions: [Fe–BC] = 1.09 g/L, [PI] = 3.29 g/L, [TC] = 20.3 mg/L, pH = 3.0, T = 25 °C, [TBA]:[PI] = 1,000:1, [FFA] = 0.2 M, and [BQ] = 5 mM.
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Inhibition by quenching agents. Conditions: [Fe–BC] = 1.09 g/L, [PI] = 3.29 g/L, [TC] = 20.3 mg/L, pH = 3.0, T = 25 °C, [TBA]:[PI] = 1,000:1, [FFA] = 0.2 M, and [BQ] = 5 mM.

Figure 6
Inhibition by quenching agents. Conditions: [Fe–BC] = 1.09 g/L, [PI] = 3.29 g/L, [TC] = 20.3 mg/L, pH = 3.0, T = 25 °C, [TBA]:[PI] = 1,000:1, [FFA] = 0.2 M, and [BQ] = 5 mM.
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Inhibition by quenching agents. Conditions: [Fe–BC] = 1.09 g/L, [PI] = 3.29 g/L, [TC] = 20.3 mg/L, pH = 3.0, T = 25 °C, [TBA]:[PI] = 1,000:1, [FFA] = 0.2 M, and [BQ] = 5 mM.

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Degradation pathways of TC

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Possible TC intermediates were identified by high performance liquid chromatography-mass spectrometry (HPLC-MS), and several products were successfully analyzed. These products were speculated to reveal the degradation pathways of TC. The specific identification results and degradation pathway speculation are shown in Figure 7. In pathway 1, TC (m/z = 445) generated P1 (m/z = 429) by demethylation and carboxylation, then P1 generated P2 (m/z = 368) by demethylation, dehydration, and deamination, then P2 generated P3 (m/z = 340) by ring-opening and hydroxyl rearrangement, and finally P3 generated P4 (m/z = 227) by dehydroxylation and dehydration. In pathway 2, TC (m/z = 445) generated P5 (m/z = 428) by dehydration reaction and then, P5 generated P6 (m/z = 403) by demethylation and dehydration reaction, followed by P6 deamination reaction to generate P7 (m/z = 384), and finally, P7 generated P8 (m/z = 325) by the dehydration reaction. In pathway 3, TC (m/z = 445) generated P9 (m/z = 396) through deamination and decarbonylation reactions, then P9 generated P10 (m/z = 314) through demethylation deamination, dehydration, followed by P10 carboxylation, demethylation to generate P11 (m/z = 309), and finally, P11 generated P12 (m/z = 279) through dehydration and dehydroxylation reactions. The end products of all three pathways were CO2, H2O, and ⁠, etc., and the mineralization of organics was achieved. The reactive species such as ·OH, 1O2, and played an important role in the degradation of TC. They had oxidizing properties in the reaction and were electron acceptors for TCs and even intermediates of TCs.
Figure 7
Possible TC degradation pathways.
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Possible TC degradation pathways.

Figure 7
Possible TC degradation pathways.
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Possible TC degradation pathways.

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In this paper, a Fe–BC was prepared using waste wine lees biomass raw material and potassium pertechnetate and used to activate PI in the treatment of antibiotic TC. The performance, influencing factors, and mechanism of TC degradation by PI systems were investigated, and the following conclusions were drawn:

  • (1) Unlike pristine BC, Fe–BC had a stronger ability to activate PI on removing TC. Under the same conditions, Fe–BC could remove 99.2% of TC, which was 16.6% higher than BC.

  • (2) After the optimization of PI concentration, Fe–BC dosage, and TC concentration by the RSM, the optimum conditions were obtained as follows: [PI] = 3.29 g/L, [Fe–BC] = 1.09 g/L, and [TC] = 20.3 mg/L.

  • (3) Cl−, ⁠, ⁠, and HA could slightly inhibit TC degradation in the Fe–BC/PI system at the given range in this study due to their quenching effects on ROS.

  • (4) Using quenching experiments, ROS in the Fe–BC/PI reaction system were identified, and ·OH, 1O2, and were considered responsible for TC removal. Moreover, TC could be degraded into final products via three pathways.

  • (5) Fe–BC was reused four times, and TC removal was above 80%, suggesting a stable performance of Fe–BC.

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The authors thank the Department of Education of Jilin Province (No. JJKH20241674KJ and JJKH20241679KJ) for the financial support.

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

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The authors declare there is no conflict.

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