The activation of persulfate by an iron-based catalyst presents a promising approach for the degradation of antibiotics; however, the activation efficiency remains a challenge. Herein, a sulfur-modified iron-based catalyst (S–Fe) by co-precipitation of sodium thiosulfate and ferrous sulfate in a molar ratio of 1:2 was prepared and the efficacy of the S–Fe/PDS system for the removal of tetracycline (TCH) was studied, where a higher removal efficiency was observed compared to the Fe/PDS system. Moreover, the effects of TCH concentration, PDS concentration, initial pH, and catalyst dosage on TCH removal were evaluated, and the highest efficiency was about 92.6% within a 30 min reaction time using a catalyst dosage of 1.0 g/L, a PDS dosage of 2.0 g/L, and a solution pH value of 7. The products and degradation pathways of TCH were analyzed by LC–MS. The free-radical-quenching experiments revealed that both SO4− • and •OH radicals contributed to the degradation of TCH in the S–Fe/PDS system, with the former playing a more significant role. The S–Fe catalyst also showed good stability and reusability for the removal of organic pollutants. Our findings suggest that the modification of an iron-based catalyst offers an effective approach to activate persulfate for removal of tetracycline antibiotics.
The sulfur-modified iron-based catalyst (S–Fe) was prepared.
Both SO4− • and •OH radicals played the role in the degradation of TCH in the S–Fe/PDS system.
The S–Fe catalyst showed high catalytic activity and reusability for the removal of organic pollutants.
Microorganisms produce antibiotics through metabolic activities that have been comprehensively utilized to treat various biological diseases (Shen et al. 2020). Among the numerous antibiotics used in industrial production and daily use, tetracycline comprises a significant proportion (Nasseri et al. 2017). However, the prevalence of tetracycline antibiotics in the environment is concerning, as they may enter water and soil environments via drug abuse and incomplete organism absorption, leading to environmental contamination (Weidner et al. 2021). Thus, the development of simple and effective treatment technologies for the removal of tetracycline antibiotics is a pressing priority.
Conventional techniques employed by sewage treatment plants primarily target the elimination of nitrogen, phosphorus, and total organic carbon present in wastewater, but not antibiotics (Kovalakova et al. 2020). Numerous approaches have been employed for the removal of tetracycline, including coagulation sedimentation, adsorption (Acosta et al. 2016), membrane separation (Li et al. 2004), biological treatment (Abejon et al. 2015), ozone oxidation (Iakovides et al. 2019), electrochemical oxidation (Borisova et al. 2013), photocatalytic oxidation (Saadati et al. 2016), and Fenton oxidation (Ma et al. 2005). Among these, advanced oxidation technology based on persulfate (PS) has garnered significant attention due to its high efficiency, ease of operation, and low cost. By breaking the O–O bond, peroxymonosulfate (PMS) or peroxydisulfate (PDS) generates sulfate radical () and hydroxyl radical (), augmenting its oxidative degradation capabilities toward organic constituents. Compared with traditional PDS activation methods such as UV- (Wacławek et al. 2017), thermal- (Zhang et al. 2019), alkali- (Qi et al. 2016), activated carbon- (Forouzesh et al. 2019), and transition-metal activation (Zhang et al. 2015), sulfur-modified iron-based materials have been proved to significantly increase the catalytic activity of PDS and PMS activation, and avoid secondary pollution (Ling et al. 2022). The enhancement of catalyst reactivity through sulfur modification could be attributed to the promotion of Fe3+ reduction to Fe2+ and the acceleration of electron transfer between the oxidant and catalyst (Wang et al. 2020). Notwithstanding the favorable catalytic properties of sulfur-modified iron-based materials, there remains a need for further improvements in the stability and magnetic properties to enhance their practical applications (Zheng et al. 2019). Therefore, it is worth developing a sulfur-modified iron-based catalyst with good reusability and stability as well as high efficiency in activating PDS for removal of organic pollutants.
In this study, the sulfur-modified iron-based catalyst (S–Fe) was prepared through co-precipitation of ferrous sulfate (FeSO4) and sodium thiosulfate (NaS2O3). The degradation of tetracycline via activated peroxodisulfate, utilizing the S–Fe catalyst, while also the effects of catalyst dosage, pH, and inorganic anions were investigated. Additionally, the involved activation mechanism was studied via quenching experiments. The results will propose a potential alternative method for effective removal of antibiotics from wastewater.
MATERIAL AND METHODS
Reagents and chemicals
Tetracycline hydrochloride (TCH, purity 96%) was obtained from Aladdin Biochemical Technology Co. (Shanghai, China). Na2S2O3 (AR), FeSO4 (AR), tert butanol (TBA, AR), sodium persulfate (Na2S2O8, AR), sulfuric acid (H2SO4, AR), sodium chloride (NaCl, AR), sodium sulfate (NaSO4, AR), and sodium carbonate (NaCO3, AR) were obtained from Sinopharm Group Chemical Reagent Co. (Shanghai, China). Sodium hydroxide (NaOH, AR), sodium nitrate (NaNO3, AR), and methanol (MeOH, AR) were obtained from Nanjing Chemical Reagent Co. (Nanjing, China). The solutions used in this study were prepared with ultrapure water.
Na2S2O3 and FeSO4 according to the molar ratio of sulfur to iron of 1:2 were dissolved with 100 mL of ultrapure water, as the most optimal yield was achieved at a ratio of approximately 1:2 according to our previous study. Meanwhile, several drops of 10% dilute H2SO4 were added during the dissolution process to prevent oxidization of ferrous ions (Fe2+). A magnetic stirrer was utilized to stir the mixture for 30 min, and subsequently, NaOH (1 mol/L) was added to adjust the pH to approximately 7. The resulting solution was subjected to filtration through a vacuum suction filtration device, and the filter residue was repeatedly washed with ultrapure water until clear liquid was obtained. The residue was air-dried at 70 °C, then calcined for 1 h in a muffle furnace at 40070 °C before it was ground into a powder. The resulting catalyst was labeled as S–Fe. The pHzpc of S–Fe was about 7.2. For the test of reusability, the catalyst was collected and cleaned with water using a vacuum suction filtration method.
Scanning electron microscopy (SEM) using a ZEISS Sigma 300 (Zeiss, German) was employed to scrutinize the shape structure, pore size, and surface morphology of S–Fe catalyst. The crystal structures of the samples were analyzed by X-ray diffractometry (XRD) with a Rigaku Ultima IV (Japan), using the Cu-Kα diffracted ray source at 2θ. Furthermore, the element composition of the material surface and the valence distribution of the target element were investigated by using X-ray photoelectron spectroscopy (XPS) with an AXIS UltraDLD (Kratos, UK), employing Al-Kα as the excitation source. The working current and voltage for the experiment were 16 mA and 12.5 KV, respectively.
Catalytic degradation experiments
All batch experiments were conducted using 250 mL conical flasks that were placed in a shaker and kept at a constant temperature of 25 °C and an agitation rate of 160 rpm. TCH, Na2S2O8, and S–Fe were added to the conical flasks as the reactants. The effects of TCH initial concentration, PDS concentration, catalyst dosage, and pH on the degradation of TCH by the S–Fe/PDS system were evaluated (Zheng et al. 2019; Ling et al. 2022). The TCH concentration was set to 20 mg/L, the dosage of S–Fe at 1.0 g/L, the PDS concentration at 2.0 g/L, and the initial pH at 7 during the radical quenching experiment. MeOH or TBA at a volume of 10 mL was also introduced into the reaction system. At predetermined time intervals, 1 mL of the sample was collected and immediately mixed with 1.0 mL of a methanol solution to quench reactive radicals. Afterwards, the samples were immediately filtered through 0.22 μm syringe filters to remove solid impurities. The concentration of residual TCH in the solution was determined by an ultraviolet visible spectrophotometer at a wavelength of 358 nm. The intermediate degradation products of TCH were determined by liquid chromatography-tandem mass spectrometry (Thermo, USA) with a C18 water column (4.6 mm × 150 mm, 5 μm) and a DAD detector at 357 nm. The mobile phase was composed of acetonitrile and 0.1% formic acid with a volume ratio of 10–90%. The column temperature was 25 °C, the flow rate was 0.5 mL/min, and the injection volume was 10 L all at once. An electrospray ion source (EIS) was used with a capillary voltage of 3.5 kV, a capillary temperature of 25 °C, a fragment voltage of 50 kV, a desolvation flow rate of 10 L/min, and a scan range of 50–800 m/z.
RESULTS AND DISCUSSION
The XRD patterns of the S–Fe catalyst are presented in Supplementary Figure S1, revealing evident diffraction peaks at 2θ values of 17.72°, 24.3°, 33.02°, 35.68°, and 53.98°. By comparing these peaks with those of iron-based substances on α-FeO/S (JCPDS-33-0664) standard reference cards, it can be inferred that the peak at 17.72° corresponds to Fe12S11O51, 24.3° corresponds to FeO(OH), 33.02° corresponds to FeS2, 35.68° corresponds to Fe3O4, and 53.98° corresponds to FeS, indicating that sulfur was superbly loaded on the surface of the material.
Degradation of TCH in different reaction systems
Influence of different factors on the catalytic activity of S–Fe
This study aimed to investigate the impact of various factors, including initial TCH concentration, catalyst S–Fe dosage, PDS concentration, solution pH, and coexistence of inorganic anions, on the removal of TCH.
Initial TCH concentration
Catalyst S–Fe dosage
The impact of on TCH degradation is depicted in Figure 5(b). According to the findings of earlier investigations (Wu et al. 2015), can suppress the redox potential of / within the system, ultimately impeding the activation efficiency of PDS and reducing the rate of TCH removal.
PDS activation can generate various active radicals, such as , , superoxide radical (), and superoxide radical (). The identification of active free radicals in the reaction system can be achieved via capturing method, as pointed out by Lai et al. (2019). MeOH is commonly used to quench both (9.7 × 108 M−1 s−1) and (1.6 × 107–7.7 × 107 M−1s−1), while TBA is more effective in reacting with (4.8 × 108–7.6 × 108 M−1 s−1) but less efficient in capturing (4.0 × 105–9.1 × 105 M−1 s−1), as reported by He et al. (2019).
TCH degradation pathway
The intermediate degradation products of TCH were analyzed by liquid chromatography–mass spectrometry (LC–MS). Four intermediate products were observed and the degradation products corresponding to the mass spectrum were determined as C22H24N2O8 (m/z = 445), C21H22N2O7 (m/z = 415), C17H16O5 (m/z = 301), and C15H16O5 (m/z = 277).
TCH degradation pathway is shown in Supplementary Figure S2. Initially, TCH underwent hydrolysis, resulting in the formation of the molecular ion (m/z = 445) (Liang et al. 2016). Subsequently, due to the low energy of the C–N bond, the molecule lost a methyl group (Kemmou et al. 2018) and a hydroxyl group, ultimately forming a substance with a mass of m/z = 415. Furthermore, free radicals attacked the electron-rich conjugated double bond in the substance with a mass of m/z = 415, resulting in the generation of the next substance (m/z = 301). Lastly, ring opening cracking of the material ion occurred, yielding the final product with a mass of m/z = 277. Considering the mineralization effect of the S–Fe catalyst on TCH, it is plausible that the products can undergo further decomposition into smaller molecules, ultimately being mineralized into CO2 and H2O.
Reusability of S–Fe
To evaluate the practicability of the catalyst S–Fe, circulation and stability studies were performed. The results indicated that most of the S–Fe catalyst could be adsorbed on a magnet, indicating its magnetism and feasibility for recycling. The degradation rate of TCH within 30 min exhibited a decrease from 92.1% in the first recycling to 91.4% in the second recycling and further decreased to 72.2% in the third recycling, thereby confirming the catalyst's good reusability and stability for organic pollutant removal. The decrease in the degradation rate during the third cycle may be attributed to the loss of catalyst during the cleaning process, continuous precipitation of metal ions in the reaction leading to a decrease in the catalyst activity (Zheng et al. 2019). Additionally, a small amount of TCH might have adsorbed onto the surface of the S–Fe catalyst, which could have decreased the surface area of the catalyst and the number of reaction active sites, leading to a subsequent reduction in catalyst activity (Du et al. 2016).
The preparation of a sulfur-modified iron-based catalyst (S–Fe) was carried out by co-precipitation of sodium thiosulfate and ferrous sulfate in a 1:2 molar ratio. The TCH removal efficiency of the S–Fe/PDS system was found to be higher than that of the Fe/PDS system, with a removal efficiency of approximately 92.6% within 30 min under appropriate conditions, indicating that the catalytic activity could be enhanced by sulfur modification of the iron-based material. Moreover, the S–Fe catalyst exhibited high reusability and stability for removing organic pollutants from wastewater, highlighting the potential application of sulfur-modified iron-based materials as persulfate activators in wastewater treatment.
This work was supported by the National Natural Science Foundation of China [grant number 42077323]. The authors also want to thank Dr. Peter Edwards from Purdue University for language editing for this paper.
CREDIT AUTHOR STATEMENT
S.G.: Data curation, software, writing – original draft preparation, visualization. N.Z.: investigation methodology, validation. L.C.: Conceptualization, writing – reviewing and editing, supervision.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.