The raw goat manure was obtained from the local farm in Yunnan. The collected goat manure was rinsed with deionized water and dried at 60°C for subsequent use. Ferrous sulfate (FeSO₄·7H₂O) and tetracycline (TC) were purchased from Aladdin Chemistry Co. Ltd., China. Other compounds including 2KHSO₅·K₂SO₄·K₂SO₄ (PMS), sodium chloride (NaCl), sodium bicarbonate (NaHCO₃), sodium nitrate (NaNO₃), ethanol (Et-OH), and tert-butanol (TBA) were supplied by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All reagents were of analytical grade and used directly without further purification. The aqueous solution involved in the experiments was prepared with deionized water produced in the laboratory (Jianghui et al., 2022).

Fe-GMC was prepared as follows: first, 20 g of goat manure was immersed in the Fe²⁺ solution (0.05 M) under continuous stirring overnight, followed by washing twice with deionized water and drying at 60°C (Luo et al., 2020). Afterward, the obtained samples were placed in the quartz boat and carbonized in a tube furnace at 800°C (at a rate of 5°C/min) for 2 h under the continuous nitrogen atmosphere. The furnace was allowed to cool, and the sample was removed and placed inside a sample bag. The prepared sample was referred to as Fe-GMC. Raw goat manure (GMC) was prepared without iron.

X-ray diffraction (XRD, Bruker D8) was conducted to confirm the crystal structure of Fe-GMC and GMC. The morphologies and structural features of Fe-GMC and GMC were determined via scanning electron microscopy (SEM; Zeiss Sigma HD, Germany) with an X-ray energy dispersive spectrometer (EDS). High-resolution transmission electron microscopy (TEM, FEI Talos F200x, United States) tests were utilized to observe the micro shape on catalyst surface. The surface elements of the Fe-GMC were recorded by X-ray photoelectron spectroscopy (XPS; ESCALAB250Xi, Thermo Fisher). Fourier transform infrared spectroscopy (FT-IR) was utilized to monitor the functional groups of the composites. Electron paramagnetic resonance (EPR, JEOL JES-FA200) examined the main active species. Brunauer–Emmett–Teller (BET) surface area was employed to obtain the specific surface area.

Fe-GMC was ground into powder and used in the batch catalytic experiments. In brief, the reactions were carried out in the 125 mL serum bottle TC (10 mg/L, 50 mL) and PMS (1.0 mM), which were initiated by adding 0.1 g of catalyst into the TC solution, which was shaken by a thermostatic reciprocating shaker at 150 rpm and 30°C. All serum bottles were covered with tin foil. At different elapsed times, a 2 mL sample was collected from each flask using a 10 mL syringe. After that, the remaining concentration of TC was measured using a UV–Vis spectrophotometer at 357 nm. The reaction kinetics of TC was simulated using the pseudo-first kinetic model. Details related to the model are provided as Text S1 in Supporting Information.

The key factors affecting the degradation of TC in the Fe-GMC/PMS system, such as initial pH, PMS dosage, Fe-GMC dosage, inorganic ions, and reaction temperature, were determined and detailed in SI (Supporting Information, see Text S2). In quenching experiments, Et-OH (SO₄ ^•− and ^•OH), TBA (SO₄ ^•−), and L-histidine (¹O₂) were selected as quenchers to scavenge corresponding active species. Afterward, the process was monitored using the UV–Vis spectrophotometer.

Figure 1A shows that the surface morphologies and structure of materials were confirmed via SEM. The SEM results determined that goat manure without supported iron exhibited a smooth surface with an irregular multilayer lamellar structure and some stripes (Figure 1A). In contrast, for Fe-GMC, some pores and ridges can be seen in Figure 1B, which increases the active sites and is more conducive to the adsorption of TC (Mingfei et al., 2021). The detailed elemental distribution and content of Raw goat manure and Fe-GMC were determined by EDS (Figures 1C, D). The element Fe was detected, which indicated the successful introduction of Fe species. Uniformly distributed Fe nanoparticles can be seen in Figure 1E, demonstrating that Fe particles are encapsulated within the GMC. As indicated by the high-magnification TEM image (Figure 1F), a thin layered structure of carbon nanoflakes could be observed, with a lattice spacing of 0.25 nm attributed to Fe₃O₄, similar to previous studies (Wang et al., 2020). In addition, the layered nanoflake structures are highly porous and become gradually thinner toward the edges of the material. Moreover, these uniformly distributed Fe₃O₄ particles onto the biochar carrier corroborated the results of SEM images of Figure 1B, and this study demonstrates the successful synthesis of Fe-GMC composites.

The surface functional groups were investigated by FT-IR. As observed in Figure 2A, the stretching vibration of C=O and C=C bonds transformed from aromatic rings results in a signal at 1,623 cm⁻¹ (Pu et al., 2021). The C–C stretching vibration was observed at 1,172 cm⁻¹, and the band at 1,084 cm⁻¹ corresponded to the C–O stretching vibration (Tianyi et al., 2020). The pyrolysis of organic to obtain the biochar played an important role in increasing oxygen-containing surface functional groups, and this might enhance the biochar activation effect (Dong et al., 2017). In addition, there was an obvious vibration at approximately 570 cm⁻¹ for Fe-GMC, which indicated the existence of an Fe-O characteristic peak in Fe-GMC, which also suggested that Fe²⁺ was successfully synthesized on the GMC (Jayaselvan L et al., 2020). In summary, the Fe-GMC had abundant oxygen-containing functional groups and oxides of Fe (II)/Fe (III). Furthermore, the crystal phase composition of the composite was determined by XRD (Shuzhao et al., 2020). Figure 2B shows a diffraction peak of Fe-GMC with a 2θ value of 43.74, 44.76, and 45.1, which corresponds to Fe₃C (Tang et al., 2021; Chen et al., 2022). In addition, the dominant Fe₃O₄ diffraction peaks were detected at 2θ values of approximately 35.49 plane (Ikramullah and Zulkarnain, 2019). This result indicated that Fe-GMC may be loaded Fe₃O₄. The diffraction peaks at 26° and 60° are assigned to Fe₂SiO₄ and magnetite, respectively, and the inorganic mineral composition Fe₂SiO₄ was detected in Fe-GMC. This might be due to the redox reaction between Fe and Si components during high-temperature pyrolysis, and there is a significant increase in the XRD peak after the composite, indicating the successful composite of Fe₃O₄ (Song et al., 2022).

Similarly, we analyzed the specific surface area of the loaded material, as shown in Supplementary Figure S1, and the Fe-GMC composites exhibit type IV adsorption curves and H3 type hysteresis loops, indicative of mesoporous attributes. In addition, the material has a good specific surface area and pore structure, with experimental results indicating a specific surface area of 416.248 m²/g. This finding summarizes the physical properties of the material, making it suitable for adsorption, catalysis, and composite superiority of the material.

To further analyze the material’s surface chemical compositions, XPS analysis was conducted. As shown in Figure 3, the XPS full-spectrum and the spectra of Fe 2p, C 1 s and O 1 s in the Fe-GMC sample were obtained. The XPS spectra (Figure 3A) confirmed the presence of C (284.8 eV) and O (531.8 eV). Compared with the full spectrum of GMC, Fe-GMC exhibited a distinct characteristic peak at 724.6 ev, which is the characteristic peak of Fe, consistent with the results of EDS characterization (Figure 1C). In Figure 3B, Fe 2p spectrum shows peaks at 710.45 eV and 724.1 eV (due to Fe 2p_3/2) and 711.85 eV and 726.3 eV (corresponded to Fe 2p_1/2), while the peaks at 718.5 eV and 733.0 eV were the satellite peaks of Fe 2p_3/2 to Fe 2p_1/2, respectively (Mohammadi and Saied Saeed Hosseiny, 2020). The peak area ratio of Fe²⁺ to Fe³⁺ obtained from the XPS analysis was close to 1:2. This result was also consistent with the abovementioned XRD analysis results, which verify the presence of Fe₃O₄ particles on the surface of Fe-GMC.

To facilitate the differentiation of the materials prior to and following the composite formation for further study, C 1 s and O 1 s spectra of biochar and the composites are separated into several fine peaks. From Figures 3C,D, it can be concluded that with the addition of Fe₃O₄, the positions of the peaks of the oxygen-containing functional groups in the composites are generally to the left, and the binding energy increases. It can also be observed from Figure 3D that the relative area of C-OH in Fe-GMC decreases, while the relative area of Fe-O increases, indicating that the oxygen-containing functional groups on the GMC play important roles in the composite of Fe₃O₄. These findings are similar to those of previous studies (Song et al., 2019). At the same time, the electron-rich groups C=C and C=O can favor the activation of PMS to produce free radicals, and these free radicals may facilitate TC removal (Chen et al., 2020; Luo et al., 2020; Zou et al., 2019). These conclusions can be a good proof of the success of the material composite.

To identify the mechanism of TC degradation in the Fe-GMC/PMS system, the contribution of related radicals in the system was differentiated by different quenching experiments (Guangfei et al., 2020). According to previously reported studies, alcohols with α-H (such as Et-OH) are capable of reacting with both ^•OH and SO₄ ^•−. TBA was employed as a radical ^•OH scavenger. L-histidine (L-His) was used to prove the existence of ¹O₂, and p-benzoquinone (p-BQ) was used to prove the existence of O₂ ^•− radicals (Luo et al., 2020). As shown in Figure 7A, the reaction was significantly inhibited when Et-OH was added to the system. Only 27.7% of TC was removed within 60 min. Furthermore, TBA also showed an intense inhibition on TC degradation, and the removal rate was 37.9%. In addition, to consider the possible presence of ¹O₂ and O₂ ^•− in the system, p-BQ and L-His were separately added into the solution, both of which showed slight inhibitions on the oxidation process. Furthermore, p-BQ and L-His were added to the solution to verify the presence of ¹O₂ and O₂ ^•−, respectively, which did not show any inhibition of TC degradation. These results indicated that SO₄ ^•− and ^•OH were involved in the activation of PMS, and ^•OH was the main contributor. In general, due to the acidic environment, ^•OH should not theoretically dominate in the Fe-GMC/PMS systems.

Fe(IV) has been reported as an emerging and powerful oxidant with higher selectivity compared to SO₄ ^•− and ^•OH(Wang et al., 2022). To further verify the generated radicals in the degradation reaction, an ESR test was coupled with 5,5-dimethylpyrroline (DMPO) and 2,2,6,6-tetramethyl-4-piperidinol (TEMP) as a spin trappers. In Figure 7B, the representative signals of 1:2:2:1 were observed, which represented DMPO-OH adducts. The formation of DMPO-^•OH adducts may also result from the formation of Fe(IV) to transform DMPO. Since the DMPO-SO₄ ^•− adduct can translate to DMPO-OH adduct, the DMPO-SO₄ ^•- adduct signal was not detected (Timmins Graham S et al., 1999). In addition, no TEMP-¹O₂ and DMPO-O₂ ^•−adducts were detected in the Fe-GMC systems, which was consistent with the results of the L-His and p-BQ quenching tests (Figure 7A). These results might be caused by the involvement of Fe(IV) in the Fe-GMC/PMS system (Equations 9–14) (Xiaowan et al., 2020; Shao et al., 2022). Fe 3 O 4 + 8 H + → Fe Ⅱ + 2 Fe Ⅲ + 4 H 2 O , (9) Fe Ⅱ + HSO 5 ‐   →   Fe Ⅳ O 2 + + SO 4 2 — + H + , (10) 2 Fe Ⅳ O 2 + + HSO 5 ‐   →   2  Fe Ⅲ + SO 4 2 — + OH ‐ + O 2 , (11) Fe Ⅱ + HSO 5 ‐   →  Fe Ⅲ + O • H + SO 4 2 — , (12) Fe Ⅱ + SO 4 • —   →  Fe Ⅲ + SO 4 2 — , (13) Fe Ⅲ + HSO 5 −   →  Fe Ⅱ + SO 5 • — + H + . (14)

Furthermore, we postulated the reaction mechanism underlying the removal of TC by •OH radicals. It is established that the TCs are susceptible to attack by reactive radicals such as •OH, O₂ ^•-, and ¹O₂ because they have three high-electron-density functional groups, namely, double bonds, phenolic groups, and amino groups. Supplementary Table S4 (Zhang et al., 2023) After the attack of •OH radical, TC will undergo demethylation and dehydration to produce intermediates 1 and 5 (Supplementary Figure S7), while intermediate 1 in the left pathway will continue to dehydration to produce 2, and the central carbon chain of intermediate 2 will break to produce intermediates 3 and 4, which will ultimately be degraded to CO₂ and H₂O. Meanwhile, in the right pathway, •OH will attack the para or ortho position of the phenol ring of 2, and demethylation occurs to yield intermediate 6, which will undergo the reaction of ring-opening and de-hydroxylation to obtain intermediate 7, which ultimately produces CO₂ and H₂O.

To confirm the charge transfer between the catalysts, we conducted electrochemical tests on both GMC and Fe-GMC materials, as illustrated in Supplementary Figure S8. Upon the addition of PMS at 100 s, a notable alteration in current was observed for both GMC and Fe-GMC, indicative of electron transfer between the catalyst and PMS. Concurrently, the addition of TC to the system at 200 s resulted in a notable decline in the current of the Fe-GMC material compared to the GMC material. This observation suggests that the Fe-GMC/PMS system facilitated the acceleration of electron transfer.