Enhanced efficiency of refractory organic pollutant degradation over a wide pH range by peroxymonosulfate activated by cobalt-doped FeS

doi:10.21203/rs.3.rs-3196793/v1

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Enhanced efficiency of refractory organic pollutant degradation over a wide pH range by peroxymonosulfate activated by cobalt-doped FeS

https://doi.org/10.21203/rs.3.rs-3196793/v1

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published 01 Feb, 2024

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A series of cobalt-doped FeS (x% Co-FeS) nanoparticles prepared using a hydrothermal method were introduced as catalysts to generate powerful radicals from peroxymonosulfate (PMS) for endocrine disrupter bisphenol S (BPS) degradation in wastewater. The kinetic results showed that Co-doped FeS substantially enhanced the catalytic performance concerning FeS in activating the oxidative degradation of BPS by PMS, and activation efficiency increased with the proportion of Co-doping. The pH was controlled with a 50 mM phosphate buffer, and over 95% of BPS (20 mg/L) was removed within 20 min at pH 6 by 7% Co-doped FeS. Moreover, exceptional activation was obtained over a wide pH range (pH 4–11). Degradation efficiency increased with increasing catalyst loading and PMS concentration, and different background ions and temperatures minimally affected BPS degradation, implying suitability for different sewage environments. Furthermore, quenching experiments coupled with electron paramagnetic resonance technology identified singlet oxygen (¹O₂) and sulfate radicals (SO₄^•−) as the primary reactive oxygen species for BPS degradation. In addition, the catalysts presented favourable cyclability and stability in repeated batch experiments, providing insights into the application of Co-doped FeS as a promising heterogeneous catalyst for removing refractory organic contaminants in Fenton-like systems.

Heterogeneous Fenton

peroxymonosulfate

Co-doped FeS

Bisphenol S

In recently, advanced oxidation processes (AOPs) based on sulfate radicals (SO₄^•−) have more widespread application prospect for organic wastewater treatment than hydroxyl radical (OH^•)-based AOPs due to their superior oxidation potential, better selectivity, longer half-life, and a wider range of pH adaptation (Lin &Zhang 2017). The primary sources of SO₄^•− in AOP systems are currently peroxymonosulfate (PMS, HSO₅⁻) and persulfate (PS, S₂O₈²⁻) (Nie et al. 2019). However, PMS typically activates easier than PS, likely due to its asymmetric structure (Chen et al. 2012).

Various conventional methods for PMS activation have been established, such as homogenous and heterogeneous transition metals (Bai et al. 2020, Duan et al. 2023, Shao et al. 2022), carbon materials (Solis et al. 2020, Yang et al. 2016), heat (Fan et al. 2015), UV irradiation (Rodriguez-Chueca et al. 2018), microwave (Qi et al. 2014), and ultrasound (Hao et al. 2014). Transition metal ions has advantages in terms of high efficiency, cost-effectiveness, and ease of operation among these activation methods. However, this method also presents some disadvantages, including the high usage rate of metal ions, difficulty in recycling, and large dependence on pH, which have yet to be resolved (Li et al. 2018). Consequently, there is a growing interest in developing secure, efficient, and reusable heterogeneous transition metal catalysts (Dong et al. 2020, Li et al. 2021). Due to low cost, non-toxicity, and abundant resources of Sulfur-containing iron minerals, they have garnered attention as catalysts for PMS activation (Hou et al. 2022). Ferrous sulfide (FeS) occurs widely in nature as a former metabolite of sulfate-reducing microorganism under environmental conditions (Ikogou et al. 2017). In recent years, FeS has been widely utilized as a catalyst for activating PMS, demonstrating good activation ability (Hong et al. 2021). Because FeS has versatile chemical properties and electronic structures, allowing it to function as both a source of Fe(II) and an electron donor(Fan et al. 2018). For example, dissolved Fe²⁺, surface Fe²⁺, and lattice Fe²⁺ in FeS both can be used as activators in sulfate radicals based AOPs (Oh et al. 2011). Moreover, S(-II) in the FeS structure and dissolved S(-II) from FeS, as an electron donor, both can reduce Fe(III) to Fe(II), significantly contributing to the Fe²⁺/Fe³⁺ cycle (Cheng et al. 2020, Cheng et al. 2016). Hence, FeS is a promising candidate for PMS activation owing to these properties.

Mackinawite and pyrrhotite, which are two different FeS phases, are imperfect catalysts (Gao et al. 2018). Pyrrhotite, being in a stable phase of FeS, exhibits a low ability to activate peroxides. In contrast, mackinawite, a sub-stable phase of FeS, has low stability and tends to dissolve to produce ferrous ions and H₂S under acidic conditions (Pankow &Morgan 1979). Gao et al. investigated the use of zinc-modified pyrrhotite and found that the addition of zinc compounds gradually transformed pyrrhotite to pyrite, resulting in a significant increase in catalytic activity (Gao et al. 2018). Because it is chalcophilic, FeS can trap various divalent metals in its natural setting by generating surface complexes, insoluble metal sulfides, or isomorphous substitution (Cheng et al. 2016). As a result, using FeS doped with transition metals may also be a viable option to increase its catalytic efficiency.

The combination of Co²⁺ and PMS is the most effective activation strategy compared to other metal ions (Anipsitakis &Dionysiou 2004, Xia et al. 2022, Yu et al. 2023). In addition, studies have demonstrated that incorporating Co-doping in FeWO₄ (Zhang et al. 2023) and Fe₃O₄ (Li et al. 2018) can significantly enhance the PMS activation, and the presence of Fe can effectively reduce the heavy metal pollution caused by the leaching of Co due to Fe-Co interactions (Liu et al. 2020, Luo et al. 2021). In natural environments, Co frequently replaces Fe in pyrrhotite owing to its affinity for sulfur and similar ionic radius to iron (Dehaine et al. 2021, Savinova et al. 2023). Various genetic types of pyrrhotite contain Co as an example of this phenomenon (Dehaine et al. 2021, Khodadadmahmoudi et al. 2022). Therefore, we predict that Co isomorphism replacing FeS may serve as a highly reactive, stable, and recyclable catalyst for PMS activation.

In this study, we utilised a hydrothermal method to prepare various Co-doped FeS catalyst materials. We used several techniques, such as field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Brunauer–Emmett–Teller, to examine the surface morphology and crystal properties of the Co-doped FeS composites. With bisphenol S (BPS) employed as a model contaminant, the effect of Co-doping on the catalytic activity of FeS-activated PMS was examined. We then examined the effects of different experimental factors on BPS degradation, including the initial solution pH, PMS concentration, catalyst dose, temperature, background substances, and actual water bodies. Furthermore, we evaluated the reusability and stability of the catalyst using recycling experiments. Quenching experiments and electron paramagnetic resonance (EPR) technology were used to identify the principal reactive species in the Co-doped FeS/PMS system for BPS removal and illustrate the role of non-radical pathways in the system. This study offers potential insights into improving iron sulfide catalysts in PMS activation for the degradation of refractory organic compounds.

2.1. Chemicals and reagents

All the chemical reagents containing FeSO₄·7H₂O, CoSO₄·7H₂O, NaNO₃, Na₂HPO₄, NaCl, Na₂CO₃, HCl, NaOH, acetic acid, ethylenediamine (C₂H₈N₂), thioacetamide (TAA), peroxymonosulfate (PMS), hydrogen peroxide (H₂O₂) (30 wt%), ethanol (EtOH), tert-butanol (TBA) are from Sinopharm Chemical Reagent Co., Ltd. furfuryl alcohol (FFA), superoxide dismutase (SOD), Sodium azide (NaN₃), benzoquinone (ρ-BQ), 5,5-dimethyl-2-pyrolin-N-oxide (DMPO), 2,2,6,6-tetramethylpiperidine pyridine (TEMP), Bisphenol S (BPS), Organic Acid II (AOII), Methylene blue (MB), Rhodamine B (RhB), Diclofenac sodium (DCF), were purchased from Shanghai Aladdin Reagent Co., Ltd. The above reagents were used without any purification. Humic acid (HA) was supplied by Adrich-sigma. All the aqueous solutions were prepared by deionised (DI) water (18.2 MΩ · cm).

2.2. Catalysts preparation and characterisation

The hydrothermal synthesis method was used to synthesise Co-doped FeS with different Co/(Co + Fe) molar ratios, which was appointed as x% Co-FeS (x = 0, 2, 5, 7, 10, 15, and 20, respectively)(Gao et al. 2018). When the proportion of doped Co was further increased, new mineral phases appeared in the XRD pattern (Fig. S1). Take 7% Co-FeS sample as an example, 18.6 mM of FeSO₄·7H₂O and 1.4 mM of CoSO₄·7H₂O was simultaneously dissolved in 50 mL of DI water, and stirred until complete dissolution. Thereafter, 10 mL of ethylenediamine and 25 mL of TAA were added and stirred for 20 min to form a uniform solution. The solution was then transferred to a 100 mL Teflon-lined autoclave and hydrothermally heated at 200°C for 20 h. After cooling at room temperature (25 ± 2°C), the product was washed with ethanol four times and then with DI water five times until the conductivity was less than 20 µs·cm^− 1. After the completion of the washing process, the product was subjected to vacuum drying at 60°C for a duration of 12 hours. To obtain the catalyst powder, the solid catalyst particles were ground and subsequently passed through a 100-mesh sieve. Other x% Co-FeS samples and CoS₂ mineral were synthesized in the same way.

2.3. Catalytic performance

BPS degradation was performed in a 50 mL conical flask at 298 ± 2 K with magnetic stirring set at 400 r·min^− 1. Because the degradation rate of BPS in 50 mM pH 6 phosphate buffer was similar to that of the automatic potentiometric titrator controlling pH, phosphate was used as the buffer in all experiments (Fig. S2). To achieve the required concentrations, suitable quantities of BPS and PMS solutions were introduced into the reactor containing a 50 mM phosphate buffer. The solution pH was regulated by the addition of 0.1 M of H₂SO₄ and 0.1 M of NaOH solutions, and the reaction was initiated by introducing an appropriate quantity of catalyst into the solution. The reaction suspension was sampled at specified time intervals by withdrawing 2 mL aliquots, which were then mixed with 2 mL of ethanol to halt the reaction. The catalyst and reaction solution was separated with a 0.22 µm polyethersulfone filter.

In order to evaluate the catalyic stability of Co-doped FeS, the particles were collected by centrifugation after each run and underwent five rounds of washing using ethanol and deionised water. Following the washing process, the catalyst was dried at a temperature of 60°C for a duration of 12 hours prior to the subsequent run. The mean of duplicates was plotted for all data, and the deviation from the mean was indicated by error bars.

All experiments were performed under dark conditions by covering the container with foil.

2.4. Analysis methods

The concentration of BPS was detected through Agilent 1260 high-performance liquid chromatography (HPLC) equipped with a UV detector (Agilent Technologies, CA, USA) at 275 nm (Xu et al. 2018). A Symmetry C18 column (4.6×150 mm, 5 µm particle size) was used as the HPLC column. The mobile phase consisted of 0.1% (v/v %) acetic acid solution and methanol at a volume of 40:60 (v/v %) and a flow rate of 1 mL/min.

The BPS intermediates were identified by ultra-performance liquid chromatography quadrupole time of flight mass spectrometry (UPLC-QTOF-MS/MS) (Agilent 1290 UPLC/6540 Q-TOF). The details were provided in Supporting Information (Text S1).

The pH_pzc (point of zero charges) of Co-doped FeS was measured using a nanoparticle size zeta potentiometer (Malvern Zetasizer Nano ZS, UK). TOC (Analytik Jena, Germany) was used as the measure of Total organic carbon (TOC). The leaching of Co and Fe during the experiments was quantified using flame atomic adsorption spectrometry (Agilent DUO AA, USA). The solution pH was measured using a PHS-3C meter (Rex, China).

To identify the potential reactive species in the reaction, electron paramagnetic resonance (EPR) analysis was performed using a Bruker EMX-10/12 instrument. In this analysis, DMPO was utilized as a spin-trapping agent for capturing OH^•, SO₄^•−, and O₂^•− species, while TEMP was employed to trap ¹O₂.

3.1 Characterisation of catalysts

In Fig. 1a, the XRD patterns of the synthesised samples, with varying concentrations of x% Co-FeS samples (x = 0, 2, 5, 7, 10, 15, and 20), are shown alongside the indexed patterns of FeS-pyrrhotite, FeS-mackinawite, and CoS₂-cattierite. As per the standard cards (PDF#22-1120) and (PDF#15–0037), the peaks at 33.928° and 43.826° correspond to the (204) and (208) planes of pyrrhotite, respectively. The peaks at 17.618°, 38.957°, and 50.433° represent the (001), (111), and (112) planes of mackinawite, respectively (Fan et al. 2018, Gao et al. 2018). All diffraction peaks of the synthesised FeS sample were identified as pyrrhotite and mackinawite. As the amount of Co doping increased, FeS gradually transformed form pyrrhotite to mackinawite. The smaller atomic radius of Co²⁺ (126pm) compared to that of Fe²⁺ (132pm) is well known. Therefore, the interplanar crystal spacing decreased when Co²⁺ was doped into the FeS structure. This resulted in a rightward shift of the diffraction peaks. As depicted in Fig. 1a, the crystallinity of the magnetic pyrrhotite phase gradually reduced as the amount of Co-doping in FeS increased, and TOPAS fitting of the data illustrated that the mineral phaser transformed into mackinawite (Table S1). In addition, both ferrous sulfide phases exhibited a clear right shift (Fig. 1b), which indicated the successful substitution of Co atoms for Fe atoms in the ferrous sulfide structure. Moreover, the morphology and particle size of FeS were obviously changed after Co-doping. The FeS sample (Fig. S3a) consisted of hexagonal flake particles with a smooth surface. However, the 7% Co-FeS sample (Fig. S3b) had a blurry edge with numerous small and irregular nanoparticles on its surface. This suggests that cobalt doping might have affected the nucleation and growth of the particles. Furthermore, HRTEM and the corresponding elemental mapping images confirmed that Co-doping resulted in mineral phase transformation and homocrystalline substitution of FeS (Fig. S3d). In panel d, the (200) lattice plane of pyrrhotite and the (111) plane of mackinawite can be assigned to d values of 0.271 and 0.235 nm, respectively. Fig. S3f, g, and h demonstrate that Fe, Co, and S were uniformly distributed in 7% Co-FeS, indicating that cobalt was incorporated into the mineral structure. To confirm the elemental composition of the 7% Co-FeS composite, both EDX analysis (Fig. S3i) and dissolution experiments (Table S1) were performed, revealing the presence of Fe and Co. It was determined that Co accounted for approximately 6–7% of the sample.

3.2. BPS removal under different doping ratios

To evaluate the performance of catalysts with different doping ratios in the Fenton-like reaction, we conducted adsorption and catalytic experiments (Figs. 2a–b). In batch adsorption experiments performed without introducing PMS, we observed an increase in the adsorption of BPS from 2.05–12.54% as the doping dosage of Co on FeS increased (Fig. 2a). This increase in adsorption may be because Co-doping potentially breaks FeS particles, exposing more active sites, as demonstrated by the FESEM images (Figs. S3a–b) and SSA (Table S1). Figure 2b illustrates that the BPS removal rates were significantly enhanced for all materials after the addition of PMS, particularly Co-doped FeS, where the rate could reach 99.5% within 30 min at pH 6.0, substantially surpassing that of the FeS/PMS system (14.4%). Incorporating a small quantity of Co substantially enhanced the PMS catalytic ability of FeS. Although Co-doped FeS presented inferior catalytic ability compared to CoS₂ (Fig. 2b), it also presented the advantage of scarcely released Co ions, thereby preventing secondary pollution (Fig. S4). Notably, although PMS was considered a strong oxidant, there was a minor effect on BPS degradation (< 12%) within 30 min in the case of only PMS, indicating the strong stability of BPS and the excellent catalytic ability of Co-FeS. The activation ability did not improve significantly with an increase in the doping ratio above 7%. Thus, Considering the activation ability and doping ratio, 7% Co-FeS was used as the catalyst in subsequent experiments.

3.3 Stability and application of Co-FeS

3.3.1 Reusability and stability of catalysts

Evaluation of the long-term operation and stability of catalyst reuse is key to catalyst selection and cost reduction in heterogeneous catalysts. To further evaluate the stability of the 7% Co-FeS catalysts, continuous batch experiments of BPS degradation were conducted using identical reaction conditions. In Fig. 3a, the 7% Co-FeS/PMS system could remove over 85% of BPS in all six cycles of the experiment, and the reaction rate was consistently high (Table S2), indicating that 7% Co-FeS maintained high activity throughout. In addition, we detected no Fe or Co dissolution after the reaction in the six cycles, indicating good stability of the catalyst. Further, the XRD patterns (Fig. S5) and FESEM images (Fig S3c) of the 7% Co-FeS after several runs showed negligible morphological changes and no apparent variation in the XRD patterns, suggesting that the 7% Co-FeS catalyst exhibited superb stability in water treatment applications. Therefore, the stability experiment in the reaction recovery process further verified the high stability and catalytic activity of Co-FeS in the activation of PMS.

Moreover, to assess the universality of the 7% Co-FeS/PMS system, several typical organic pollutants such as RhB, AOII, MB, BPS, and DCF were selected for evaluation. As illustrated in Fig. 3b, the degradation efficiencies of RhB, AOII, MB, and BPS reached 100% within 15 min, whereas DCF reached 90% within 30 min. The obtained results clearly demonstrate the capability of the 7% Co-FeS/PMS system to effectively catalyze the degradation of a wide range of persistent organic pollutants.

3.3.2 Effect of co-existing substances and actual water bodies

In aqueous environments, various inorganic anions and natural organic matter are ubiquitous. Their presence can affect the efficacy of water treatment technologies by scavenging free radicals. Therefore, we analysed the impact of widespread anions, such as NO₃⁻, Cl⁻, HCO₃⁻, and typical natural organic matter HA on the degradation rate of BPS (Fig. 4). In general, PMS-based advanced oxidation techniques are inhibited by NO₃⁻, Cl⁻, and HCO₃⁻, as they react with the reactive oxygen species (ROS) generated by PMS activation, resulting in the production of species with substantially lower oxidation. For example, the generation of less reactive chlorine radicals, such as Cl^• (2.4 V), Cl₂^•− (2.09 V), and Cl₂ (1.36 V), was observed when SO₄^•− reacted with Cl⁻ [Eqs. (1)–(3)] (Nie et al. 2014, Yang et al. 2018). Concurrently, Cl⁻ generates reactive halogens (such as HOCl) when it reacts with PMS [Eqs. (4) and (5)] (Lente et al. 2009). Similar to Cl⁻, HCO₃⁻ can produce CO₃^•− with weak oxidising ability when interacting with SO₄^•− and OH^• [Eqs. (6) and (7)] (Deng et al. 2014), and when the concentration of HCO₃⁻ is sufficiently high, it could negatively impact ¹O₂ production (Tian et al. 2017). However, in the 7% Co-FeS/PMS system, these three anions had negligible influence on the BPS degradation and reaction rate (Figs. 4a–c), and the BPS removal rate was 93.3% regardless of HCO₃⁻ concentrations of 30 mM (Fig. 4c). As the most prevalent organic substance in the environment, HA may compete with pollutants for ROS in advanced oxidation techniques, resulting in decreased pollutant removal (Brian P. Chaplin 2006, Nie et al. 2014). However, in the 7% Co-FeS/PMS system, the effect of HA was minimal. After adding HA at concentrations of up to 15 mg C/L, the BPS removal rate remained above 90% (Fig. 4d), and the reaction rate was slightly reduced (Table S2). Therefore, these results indicated that the 7% Co-FeS/PMS system was highly applicable and had good generalisability.

SO₄^•− + Cl⁻ → SO₄²⁻ + Cl^•	(1)
Cl^• + Cl⁻ → Cl₂^•−	(2)
2Cl₂^•− → 2Cl⁻ + Cl₂	(3)
2Cl⁻ + HSO₅⁻ + H⁺→ SO₄²⁻ + Cl₂ + H₂O	(4)
Cl⁻ + HSO₅⁻ → SO₄²⁻ + HOCl	(5)
SO₄^•− + HCO₃⁻ → SO₄²⁻ + CO₃^•−	(6)
OH^• + HCO₃⁻ → H₂O + CO₃^•−	(7)

The consumption of ROS in advanced oxidation systems by organic or inorganic anions present in lakes and seawater is a concern. Therefore, we investigated the degradation of BPS in various water bodies to assess the practical application of the 7% Co-FeS/PMS system in actual water. As shown in Fig. S6, the degradation rates of BPS in tap water, South Lake water, East Lake water, and seawater were 98%, 97.3%, 97.7%, and 95%, respectively. The degradation rates were approximately identical to those in ultrapure water, indicating that 7% Co-FeS can maintain high catalytic performance for PMS in various water substrates, and the 7% Co-FeS/PMS system is highly versatile.

3.4. Catalytic mechanism

3.4.1. ROS identification

To determine the principal active species involved in BPS degradation, quenching experiments and EPR were performed. The contribution of free radicals to BPS degradation was measured using TBA to quench OH^• (Yao et al. 2015), EtOH to quench SO₄^•− and OH^• (George P.Anipsitakis &Dionysiou 2003), and ρ-BQ and SOD to quench O₂^•− (Chen et al. 2020, Dai et al. 2017). As could be observed from Fig. S7, BPS was approximately removed within 30 min without scavengers. The degradation of BPS, as well as the reaction rate constant, experienced a slight decrease when the solution contained 10 mM of TBA and 10 mM of EtOH. The degradation and reaction rate constant of BPS decreased considerably when EtOH was increased to 100 mM. However, when TBA was increased to 100 mM, the inhibitory effect was not distinctly increased. The aforementioned quenching results indicated that SO₄^•− and OH^• are reactive species in 7% Co-FeS/PMS but that SO₄^•− is more significant than OH^•. Furthermore, the quenching experiments demonstrated that when ρ-BQ or SOD (O₂^•− scavenger) was introduced, the degradation rate remained unchanged, although the reaction rate constant slightly decreased (Fig. 5a). This suggested that O₂^•− plays a role in BPS degradation but may not be the primary active species responsible. In addition, to verify non-radical processes, NaN₃ and FFA were employed as scavengers to quench ¹O₂ (Liu et al. 2021). When FFA or NaN₃ was added, the removal rate of BPS exhibited a decrease, reaching values of 10.1% and 10.5%, respectively, suggesting that ¹O₂ greatly contributed to the degradation process.

To further identify the results of quenching experiments, EPR measurements were performed using DMPO and TEMP as free radicals and ¹O₂ spin traps, respectively. Figures 5b–d present the observed results. As projected, the EPR experiments illustrated that DMPO-OH^•, DMPO-SO₄^•−, DMPO-O₂^•−, and TEMP-¹O₂ signals could easily be detected in 7% Co-FeS suspensions in the presence of PMS (Li et al. 2021, Liu et al. 2021). This discovery further confirms that the degradation of BPS in the 7% Co-FeS/PMS system is attributed to the involvement of various free radicals, such as OH^•, SO₄^•−, O₂^•−, as well as the non-radical ¹O₂. In conclusion, the dominant active species involved in the degradation of BPS were identified as SO₄^•− and ¹O₂.

3.4.2 Effect of Dissolved Oxygen

The influence of dissolved O₂ on the generation of free radicals in SO₄^•− -based AOPs have been reported to affect the degradation performance of pollutants (Zhu et al. 2018), but this remains controversial (Zhou et al. 2020). To investigate this, BPS degradation with air and N₂ atmospheres was conducted in the Co-FeS/PMS system (Fig. S8a). The degradation rate of BPS (97.4% vs. 97.3%) and the reaction rate constant of the system (0.144 min^− 1 vs. 0.138 min^− 1) were not significantly affected by purified N₂ compared to the aerobic environment. Based on these findings, it can be inferred that O₂ does not play a significant role in the degradation of BPS. The oxygen contents under the air and N₂ purging modes indicate that most of the ¹O₂ and O₂^•− in the reaction originated from PMS rather than dissolved oxygen (Fig. S8b). Evidence suggests that ¹O₂ can decay rapidly to triplet oxygen (³O₂) (Cheng et al. 2017, Zhou et al. 2015). Therefore, as shown in Fig. S8b, the increase of DO in both purging modes was indirect evidence for ¹O₂ production in the 7% Co-FeS/PMS system. The subsequent decrease in dissolved oxygen may be due to the reaction of ¹O₂ and O₂^•− with the organic matter.

3.4.3 Possible mechanism for PMS activation by Co-FeS

To investigate the activation mechanism of PMS with 7% Co-FeS, XPS analysis was employed to characterise the change in the valence states of Co, Fe, and S in the catalyst prior to and after the reaction (Figs. 6a–d). Fe, Co, S, O, and C were detected in the XPS survey spectrum (Fig. 6a). Figure 6b shows that there are two spin-orbit doublets and two dithered satellites in Co2p XPS spectrum. Combined with previous studies(Zhu et al. 2019a, Zhu et al. 2019b), the spin-orbit doublet of Co²⁺ was located at 782.5 eV (Co2p_2/3) and 794.9 eV (Co2p_1/2), whereas that of Co³⁺ was located at 779.6 eV (Co2p_2/3) and 789.0 eV (Co2p_1/2). Concurrently, two broad peaks of 786.0 eV and 803.4 eV were assigned to the restructured satellite. By comparing the data before and after the reaction, the percentage composition of Co(II) decreased from 56.43–28.65%, while that of Co(III) increased from 43.57–71.35%, implying that there was electron-transfer process between Co(II) and PMS during the activation. Similarly, electron transfer also occurred in Fe (Fig. 6c). The presence of Fe(II) in sample was suggested by the peaks at 707.5 eV and 711.1 eV, while that of Fe(III) was labelled as the peak at 713.4 eV (Gao et al. 2018, Zhang et al. 2023). After PMS activation, the proportion of Fe(II) decreased slightly from 76.25–68.77%, while Fe(III) correspondingly increased from 23.75–31.23%. Fe(II) remained the primary Fe species in sample after the reaction, indicating that 7% Co-FeS still retained high catalytic activity. These results agreed with the XRD patterns of the pristine and used 7% Co-FeS, which showed approximately no change in structure and crystallinity (Fig. S5). Analysis of the S2p XPS spectrum (Fig. 6d) revealed that S species at low binding energy primarily existed as S²⁻ (161.4 eV) and S_n²⁻ (163.0 eV), whereas the peak at high binding energy of 168.7 eV was assigned to SO₄²⁻ species (Fan et al. 2018). Notably, the percentage of S²⁻ decreased significantly from 14.9–6.27% after the reaction, indicating that S²⁻ acted as an electron donor and reduced Fe(III) to Fe(II) (16) (Fan et al. 2018), which was consistent with a previous study (Zhou et al. 2018). The proportion of S_n²⁻ increased, indicating that S_n²⁻ might be one of the products of S²⁻ oxidation. The decrease in SO₄²⁻ may be due to its dissolution in water.

≡ Co(II) + HSO₅⁻ → ≡ Co(III) + SO₄^•− + OH⁻	(8)
≡ Fe(II) + HSO₅⁻ → ≡ Fe(III) + SO₄^•− + OH⁻	(9)
≡ Fe(III) + HSO₅⁻ → ≡ Fe(II) + SO₅^•− + H⁺	(10)
≡ Co(III) + HSO₅⁻ → ≡ Co(II) + SO₅^•− + H⁺	(11)
4SO₅^•− + 2H₂O → 4HSO₄⁻ + 3¹O₂	(12)
SO₅²⁻ + HSO₅⁻ → HSO₄⁻ + SO₄²⁻ + ¹O₂	(13)
SO₄^•− + OH⁻ → SO₄²⁻ + OH^•	(14)
SO₄^•− + H₂O → HSO₄⁻ + OH^•	(15)
S₂⁻ + ≡Fe(III) + 4H₂O → ≡Fe(II) + SO₄²⁻ + 8H⁺	(16)
≡ Fe(II) + ≡ Co(III) → ≡ Co(II) + ≡ Fe(III)	(17)

Based on the above results and previous studies, a possible mechanism of 7% Co-FeS activating PMS in this system is illustrated as follows (Fig. S9): First, PMS was adsorbed on the surface of 7% Co-FeS due to electrostatic interactions (Fig. S11). Co(II) and Fe(II) then partially activated PMS, resulting in the generation of SO₄^•− and ¹O₂, according to Eqs. (8)–(13) (Liu et al. 2023, Liu et al. 2018, Oh et al. 2017). Subsequently, OH^• was produced by the reaction between SO₄^•− and H₂O/OH⁻, as indicated by Eqs. (14) and (15), respectively (Li et al. 2020). All these reactive species contributed to the degradation of BPS.

PMS activation requires Co(II) and Fe(II) to provide electrons, which are subsequently oxidised to Co(III) and Fe(III). Owing to the higher redox potential of Co(III)/Co(II) (1.81 V) compared to that of Fe(III)/Fe(II) (0.77 V), the Co(III) that was oxidised by PMS could be reduced by the surrounding Fe(II), thus achieving the Co(III)-Co(II) redox process [Eq. (17)] (Hu &Long 2016). Furthermore, the reduction of Fe(III) to Fe(II) was achieved by S²⁻ and S_n²⁻ acting as electron donors, as per Eq. (16) (Xu &Sheng 2021). The regeneration of Fe(II) and Co(II) can also be achieved through PMS reduction [Eqs. (10) and (11)] (Li et al. 2020, Wang et al. 2021). The co-cycling of Co and Fe enables a continuous and efficient PMS activation process.

3.5 Possible degradation pathway of BPS

To explore BPS degradation process in the 7% Co-FeS/PMS system, UPLC-QTOF-MS/MS was used to analyse and identify the degradation products of BPS, which are presented in Table S3. Based on these products, three pathways are proposed in Fig. S10.

Pathway I involved the oxidation of the phenolic hydroxyl group on the BPS molecule, resulting in the formation of an oxygen heterocycle (P1), which was further oxidised to the subsequent products (P2, P3, and P4) (Peng et al. 2020).

Pathway II was initiated by β-breaks in BPS, resulting in the formation of P5, which was then hydroxylated to produce P6. After adding hydroxy groups to the aromatic nucleus of P6, a dehydroxylation reaction occurred, resulting in the formation of P7, which is an isomer of P5 (Lv et al. 2021).

In Pathway III, the ROS reacted with the aromatic ring of BPS, and the hydrogen on the aromatic ring was extracted to form the BPS radical. The BPS radical binds to a biomolecule via C-C coupling, producing an intermediate with a higher molecular weight. Subsequently, the phenolic hydroxyl group of the intermediate is oxidised to form P8 (Lv et al. 2021, Peng et al. 2020).

Finally, the above intermediates can be broken down into smaller products or directly mineralised.

In this study, the efficiency of PMS activation by FeS is improved by Co doping, leading to the repid degradation of BPS. Investigation of the mechanism confirmed the presence of various active species in the Co-FeS/PMS system, including SO₄^•−, OH^•, O₂^•−, and ¹O₂. Among these species, SO₄^•− and ¹O₂ were found to play a significant role in the efficient removal of BPS. As an electron donor, S²⁻ on the surface of Co-FeS could continuously promote the reduction of Fe(III) and Co(III), and the reconstituted Fe(II) and Co(II) ensured the continuous and efficient activation of PMS by Co-FeS. The degradation of BPS was minimally influenced by various background ions and different temperatures. Furthermore, The Co-FeS/PMS system exhibited suitability for natural surface waters and maintained excellent degradation performance for BPS. In addition, Co-doped FeS maintained high reusability, stability, and activity after six cycles of use. In summary, the Co-FeS/PMS system exhibits characteristics of high efficiency, exceptional catalyst recovery capability, and a wide pH application range, which will have promising application prospects in organic pollutant wastewater treatment. In addition, the results provide insight into the modification of natural FeS-type minerals for environmental applications.

Author Contributions

Yanting Pan: Investigation, Experiment, Validation, Formal analysis, Writing-Original Draft; Feng Zhang: Investigation, Formal analysis, Writing – review & editing; Ziyang Zhou: Experiment; Feng Jiang: Writing – review & editing; Xiaoming Wang: Resources, Supervision, Writing – review & editing; Hui Yin: Resources, Supervision, Writing – review & editing; Wenfeng Tan: Resources, Supervision, Writing – review & editing; Xionghan Feng: Conceptualization, Resources, Writing - Review & Editing, Supervision, Project administration, Funding acquisition. All authors contributed to the study conception and design. All authors read and approved the final manuscript.

Funding

This research was supported by the National Natural Science Foundation of China (42030709).

Data availability

All data are fully available without restriction.

Competing interest

The authors declare no competing interests.

Ethical Approval

Not applicable.

Consent to Participate

Not applicable.

Consent for publication

Not applicable.

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Enhanced efficiency of refractory organic pollutant degradation over a wide pH range by peroxymonosulfate activated by cobalt-doped FeS

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Abstract

Figures

1. Introduction

2. Materials and methods

2.1. Chemicals and reagents

2.2. Catalysts preparation and characterisation

2.3. Catalytic performance

2.4. Analysis methods

3. Results and discussion

3.1 Characterisation of catalysts

3.2. BPS removal under different doping ratios

3.3 Stability and application of Co-FeS

3.3.1 Reusability and stability of catalysts

3.3.2 Effect of co-existing substances and actual water bodies

3.4. Catalytic mechanism

3.4.1. ROS identification

3.4.2 Effect of Dissolved Oxygen

3.4.3 Possible mechanism for PMS activation by Co-FeS

3.5 Possible degradation pathway of BPS

4. Conclusion

Declarations

References

Supplementary Files

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