Citation

Chao Guo, Dongting Yue, Songling Wang, Xufang Qian, Yixin Zhao. Mechanochemically sulfured FeS_1.92 as stable and efficient heterogeneous Fenton catalyst[J]. Chin. Chem. Lett., 2020, 31(7): 1978-1981  

Chao Guo, Dongting Yue, Songling Wang, Xufang Qian, Yixin Zhao^*     

Persistent organic pollutant wastewater remediation has been of vital significance for realizing a sustainable environmental remediation and natural water resource conservation [1-4]. Among various physical and chemical water treatment techniques dealing with organic pollutants in wastewater, advanced oxidation processes (AOPs) utilizing hydroxyl radicals (^·OH) as the oxidant has been intensively investigated in the past decades [5-7]. As one of the most popular AOPs, Fenton reaction has been successfully adopted in various organic pollutants control and recalcitrant compounds degradation [8-14]. However, the required acidic condition is the major drawback for Fenton reactions because of the inevitable neutralization process and secondary sludge pollution generation [10, 15-17]. Therefore, lots of research efforts have been invested into development of effective heterogeneous Fenton catalysts suitable for mild conditions, which could avoid the neutralization process and formation of secondary sludge [18-20]. Various heterogeneous Fenton catalysts including carbon nanotube-supported Fe₃O₄, nanometer zero valent iron, and metal doped iron oxide have been reported [13, 21-28]. However, these efficient catalysts processing are complicated and expensive, which is not suitable for large-scale applications.

In this report, the low cost mechanochemical method is adopted to fabricate micron size FeS_1.92 as heterogeneous Fenton catalysts. The mechanochemical approach exhibits technical advantages of small particle sizes and low agglomeration besides its low cost for large-scale fabrications [29-32]. Specifically, the synthesis of a stable sulfured microscale FeS_1.92 by ball milling mackinawite (FeS) with sulfur powder (S) is demonstrated in this work. Mechanochemically fabricated FeS_1.92 shows high catalytic activities for heterogeneous Fenton reaction under mild condition. Furthermore, this FeS_1.92 Fenton catalyst also shows high sulfide resistance and low Fe³⁺ leaching.

In mechanical milling synthesis, FeS and S powders were mixed together by different ratios. The ball milling process with intensive fracturing and grinding would promote the solid state reaction of FeS with sulfur, which lead to the formation of FeS_1.92 alloy phase [33, 34]. The X-ray diffraction (XRD) pattern of as synthesized sample (Fig. 1a) shows the peaks related to FeS, S and FeS_1.92. The peak intensity of S becomes stronger with increasing the molar ratios of S/FeS (Fig. S1 in Supporting information). This XRD results suggest that the mechanochemical synthesis can obtain the FeS_1.92 with some FeS and S remained. In order to verify the chemical structures of FeS_1.92, we then washed the sample with CS₂ to remove the free sulfur. The XRD peaks of washed FeS_1.92 exhibit characteristic FeS_1.92 peaks. The scanning electron microscope (SEM) image of FeS_1.92 sample shows a floccus morphology, which is related to the mixture of free S with FeS (Fig. 1b and Fig. S2 in Supporting information). The elemental mapping shows that the main element components of the as-synthesized catalyst are Fe and S, which are homogeneously distributed (Fig. S3 and Table S2 in Supporting information). The energy dispersive X ray spectros-copy (EDS) measurement suggests the S content in FeS_1.92 is about twice than FeS (Table S3 in Supporting information). X-ray photoelectron spectroscopy (XPS) measurement was performed to explore the chemical bond information of the Fe and S. Two main peaks of FeS sample locate at 724.9 eV and 711.4 eV, which can be assigned to 2p_1/2 and 2p_3/2 of Fe. After sulfuring treatment, these two main peaks shift to the higher banding energy (725.1 eV and 711.6 eV) (Fig. 1c). In contrast, FeS_1.92 has two 163.3 eV and 164.4 eV peaks related to S^2- and S⁰ in Fig. 1d. All these results confirm the reaction between FeS and S during the ball-milling process.

Figure 1

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Fig. 1. (a) XRD patterns of FeS raw material and obtained FeS1.92 powder before and after washed by CS2. (b) SEM image of FeS1.92. (c) Fe 2p XPS spectra of FeS and FeS1.92. (d) S 2p XPS spectra of FeS and FeS1.92.

The catalytic activation of H₂O₂ by FeS_1.92 catalyst was tested in the simulated sulfide-containing organic wastewater, which was prepared by adding Na₂S in Bisphenol A (BPA) solution under mild conditions. Fig. 2a showed the catalytic activation of H₂O₂ using different catalysts with or without [Na₂S]. Neither FeS_1.92 nor H₂O₂ alone showed any activites for BPA degradation. The regular Fenton system (Fe²⁺/H₂O₂) can remove nearly 60% of BPA in the absent of [Na₂S]. However, its removal efficiency decreased to only 10% in the presence of [Na₂S] because the Fe²⁺ would be deactivated by S^2-. Excitingly, the FeS_1.92/H₂O₂ system exhibited excellent BPA degradation activities with or without [Na₂S]. The catalyst of FeS and S, which was prepared by regular mixing and grinding of the same S/FeS molar ratio, did not present the same catalytic performance. Thus, it can be speculated that the alloy of FeS and S was formed by ball-milling, which is important for the heterogeneous Fenton reaction in sulfide-containing organic pollutants water.

Figure 2

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Fig. 2. (a) Degradation rates of BPA with FeS1.92 and control experiments. (b). Degradation rates of BPA over several catalysts with different S/FeS molar ratios. Reaction conditions: [catalyst] = 0.25 g/L, [H2O2] = 7.5 mmol/L, [BPA] = 100 mg/L, [Na2S] = 0.2 g/L, pH 6.0.

In order to optimize the S/FeS molar ratio for FeS_x catalysts, we studied the catalytic activities of several FeS_x catalysts with different S/FeS molar ratios. Fig. 2b shows the FeS_1.92 show the highest Fenton catalytic performance. It seems that the added S can increase the active sites on the surface of FeS to activate H₂O₂ to produce ^·OH for a better catalytic capacity. When the S powder was insufficient, the catalyst exhibited lower activities. However, too much S would reduce the catalytic activities because too much S will cover the active sites on the surface. The FeS_1.92 could be the optimal for FeS_x based heterogeneous Fenton catalysts. The pH value and H₂O₂ concentration have great impact on the Fenton performance of FeS_1.92. The degradation efficiency decrease with increase of pH values. The BPA can hardly be oxidized or decomposed when pH values are 8 and 10, which was attributed to the formation of iron hydroxide coating on the catalyst surface and the rapid self-decomposition of H₂O₂ under alkaline conditions [35, 36]. However, Fig. S4 (Supporting information)shows that a larger amount of Fe³⁺ leaching was observed under lower pH conditions. The pH 6–7 is the optimal condition to suppress Fe leaching and keep catalytic activity of FeS_1.92 (Fig. 3a). Here we selected the pH 6 for the heterogeneous FeS_1.92 performance evaluation. When the H₂O₂ concentration increases from 2.5 mmol/L to 7.5 mmol/L, the degradation efficiency slightly increases (Fig. 3b). Therefore, the H₂O₂ concentration was maintained at 7.5 mmol/L. Moreover, the S^2- concentration in wastewater also affects the degradation performance of FeS_1.92. Interestingly, the FeS_1.92 showed enhanced catalytic activities for BPA degradation when the S^2- concentration reached up to 0.8 g/L (Fig. 3c). Such enhancement might be ascribed to that the more Fe active sites could formed on the FeS_1.92 surface under the S^2- -rich condition because the S_n^2- accumulated on FeS_1.92 surface can promote the reduction of Fe³⁺ to Fe²⁺ [37]. When the concentration of S^2- increased up to 1.2 g/L, it began to inhibit the degradation of BPA and it is due to that the ^·OH radicals can react with the redundant S^2-, resulting in the inhibition of BPA degradation.

Figure 3

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Fig. 3. Effects of pH values (a), H2O2 concentrations (b), Na2S concentrations (c) on BPA removal efficiency. Cyclic stability of FeS1.92/H2O2 system for BPA degradation (d). Reaction conditions: [catalyst] = 0.25 g/L, [BPA] = 100 mg/L, [H2O2] = 7.5 mmol/L for a, c and d, [Na2S] = 0.2 g/L for a, b and d, pH 6.0 for b, c and d.

The stability of FeS_1.92 was also evaluated under the optimal condition with pH 6.0, 7.5 mmol/L H₂O₂ and 0.2 g/L Na₂S. After four cycles, the FeS_1.92 still showed excellent degradation performance and an even better catalytic activity (Fig. 3d). During the cycle experiment, the S^2- can be accumulated on FeS_1.92 surface, which can promote the Fe³⁺/Fe²⁺ cycle and then enhance the performance for the degradation of BPA. Additionally, the Fe³⁺ concentration in the solution after reaction was below 1.0 ppm in the repeated tests (Fig. S5 in Supporting information). No obvious morphology changes can be found in the FeS_1.92 after reaction as shown in SEM image (Fig. S6 in Supporting information). No change is obviously observed in XRD patterns of FeS_1.92 before and after the reactions (Fig. S7 in Supporting information), indicating a high chemical stability of FeS_1.92. Raman spectroscopy also showed no significant changes on FeS_1.92 surface before and after the reaction (Fig. S8 in Supporting information). However, the XPS of FeS_1.92 before and after the reaction showed that the proportion of S in FeS_1.92 decreased. This is probably because S in FeS_1.92 was consumed as a sacrificial agent during the reactions (Fig. 4a and Fig. S9 in Supporting information). These results show that FeS_1.92 is a promising catalyst for degradation of sulfide-containing organic pollutants water in heterogeneous Fenton system. We further evaluated the removal efficiency of two more organic pollutants including 2, 4, 6-trichlorophenol (TCP) and sodium benzene sulfonate (SBS). We can see that both TCP and SBS pollutants are completely degraded within 15 min, and their mineralization efficiency was relatively high (Figs. S10 and S11 in Supporting information).

Figure 4

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Fig. 4. (a) The proposed scheme of H2O2 activation by FeS1.92 (b) S 2p XPS spectra of FeS1.92 before and after reaction. (c) The concentration of SO42- in the solution after the reaction. (d) 5, 5-Dimethyl-1-pyrroline N-oxide (DMPO) spin capture EPR spectrum in different reaction systems. Reaction conditions: [catalyst] = 0.25 g/L, [H2O2] = 7.5 mmol/L, [BPA] = 100 mg/L, pH 6.0.

The degradation mechanism of organic compounds in FeS_1.92/ H₂O₂ system was investigated. As shown in Fig. 4b, the Fe²⁺ as the active sites in FeS_1.92 was easily oxidized to Fe³⁺, inducing the formation of the ^·OH radicals (Eq.1) [38-41]. Then, the ^·OH radicals can mineralize organic pollutants into CO₂ and H₂O. The S⁰ or SO₄^2- on FeS_1.92 surface, as the electron donor, can react with Fe³⁺ to form Fe²⁺ and improve the Fe³⁺/Fe²⁺ cycle (Eqs. 2 and 3) [42-44]. Compared with FeS, the chemical bonding between FeS and S in FeS_1.92 can promote the Fe³⁺/Fe²⁺ cycle for better degradation performance. When the solution contains appropriate amount of S^2-, it can also promote the conversion of Fe³⁺ to Fe²⁺ (Eq. 4) [37].

(1)

(2)

(3)

(4)

To verify this speculation, the evolution of surface S in FeS_1.92 during Fenton reaction was investigated. According to the XPS analysis, the proportion of S₀ or S_n^2- in FeS_1.92 after the reaction decreased, indicating that S was consumed to some extent in the process of the reaction (Fig. 4a and Table S3 in Supporting information). The sulfur seems to be oxidized into SO₄^2- and can be detected in the final reaction solution (Fig. 4c). Moreover, the SO₄^2- produced in FeS_1.92/H₂O₂ system is about twice as that produced in FeS/H₂O₂ system, indicating that sulfur added to FeS by ball-milling does participate in the reaction. For a better insight into the role of S in this FeS_1.92 Fenton catalyst, the effect of SO₄^2- in the Fenton reaction was investigated. The result indicated that the SO₄^2- did not impact the BPA degradation in FeS_1.92/H₂O₂ system. This result confirmed that the performance enhancement was mainly attributed to the formation of S⁰ or S_n^2- on FeS_1.92 surface (Fig. S12 in Supporting information). Electron paramagnetic resonance (EPR) analysis was also conducted to explore the above mechanism. As shown in Fig. 4d, the hydroxyl radical adducts of 5, 5-dimethyl-1-pyrroline N-oxide (DMPO-OH) (1:2:2:1) signal was observed in FeS_1.92/H₂O₂ system, indicating the generation of ^·OH radicals in the sulfide-containing solution. The relative EPR intensity of FeS_1.92/H₂O₂ system exhibit higher EPR intensity than that of FeS/H₂O₂ system. These results indicated that S in FeS_1.92 promote the ^·OH radicals generation especially in the sulfide-containing organic pollutants water.

In summary, a highly efficient FeS_1.92 based heterogeneous Fenton catalyst was synthesized by a facile ball milling of FeS with S powder. In the mechanochemical reaction, the S species can reduce Fe³⁺+ to Fe²⁺ for Fenton-like reaction. At the same time, this FeS_1.92 Fenton catalyst shows a highly sulfide resistance and improved performance for degrading sulfidecontaining organic pollutants. This work has broadened the application of FeS_x based environmental catalyst in wastewater treatment, especially for sulfide-containing organic pollutants water.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This work was supported by the National Natural Science Foundation of China (No. 21777097) and Shanghai Shuguang Grant (No. 17SG11) and the China Postdoctoral Science Foundation (Nos5. 2017M621483, 2018T110397).

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.11.049.