Promoting azo dye decomposition in natural molybdenite activated peroxymonosulfate process by low concentration of ferrous ions

In recent years, sulfate radical (⁠⁠)-based advanced oxidation processes (SR-AOPs) has attracted more and more attention in the field of refractory organic pollutant degradation, because generated from peroxymonosulfate (PMS, ⁠) or peroxodisulfate (PDS, ⁠) activation has a higher standard redox potential, longer half-life period and wider pH application range than hydroxyl radical (⁠⁠) (Kohantorabi et al. 2021). For the reason that PMS holds an asymmetric structure, it can be activated more easily than PDS (Guo et al. 2020).

Molybdenite (MDN), a natural sulfide mineral, mainly contains MoS₂ in its component, which is widely distributed in the molybdenum mineral and easy to get (Du et al. 2020). However, the research of MDN is scarce in investigations nowadays. In our previous study, MDN has been validated that it could activate PMS directly and Mo⁴⁺ on its surface provided activation sites for PMS. In addition, low-valence S in the vicinity of Mo⁴⁺ was able to reduce Mo in a high valence after reaction, which expedited the orange G (OG) degradation reaction (Yuan et al. 2021a). However, the MDN usage was 3.0 g/L in MDN/PMS system, which is a large amount in the degradation process. Furthermore, the reusability of MDN was hard to sustain in the test for that only Mo ion on MDN surface participated in the reaction and was reduced by its vicinity S. Dissolved Mo in the solution would form molybdate, which could not be reduced to Mo(IV) anymore. Now, we speculate the addition of Fe²⁺ in the solution would improve the availability of MDN in the system. To our knowledge, there is no report involving PMS activation by both Fe²⁺ and MDN.

In this work, MDN was used as co-catalyst to test its synergetic effect with Fe²⁺ for PMS activation to degrade a typical azo dye, OG. The objectives of the present work are (i) to study the synergistic linkage between MDN and Fe²⁺ in activating PMS; (ii) to explore the effects of key parameters on OG removal; (iii) to identify the generated reactive oxygen species (ROS) and possible catalyst mechanism in the system (iv) to test the reusability of catalyst and broad applicability of the system.

Unless stated otherwise, all the chemicals used in this study were analytical grade. L-Histidine (≥99%), Alizarin Red S (ARS) with biotechnology grade, orange G (OG) with HPLC grade (>96%) and p-benzoquinone (BQ) were purchased from Shanghai Macklin Biochemical Co. Ltd, China. Potassium monopersulfate triple salt (PMS, KHSO₅•0.5KHSO₄•0.5K₂SO₄, ≥47%)), L-Histidine (≥99%), MoS₂ powder (99.5%, <2 μm), molybdenum (IV) dioxide (MoO₂, 99%), molybdenum (VI) trioxide (MoO₃), molybdenum (V) chloride (MoCl₅, 99.6%), furfuryl alcohol (FFA, 98%) were supplied by Aladdin Chemistry Co., Ltd. FeSO₄•7H₂O (>99.0%), tetracycline (TC) hydrochloride, norfloxacin (NFX), rhodamine B (RhB), phenol, methanol, tert-butyl alcohol (TBA) were purchased from Sinopharm Group Chemical Reagent Co. Ltd, China. The preprocessing method of raw MDN was the same as our previous study (Yuan et al. 2021a), and its main component was MoS₂.

All experiment reactions were completed on a magnetic stirrer with constant temperature water bath at a stirring rate of 150 rpm. Batch experiments were operated in a 300 mL glass container covered with aluminum-foil paper at 30 ± 0.5 °C to prevent the effects of light. OG aqueous solution was prepared by adding a certain amount of OG into deionized water. The batch experiments were started immediately after adding Fe²⁺, PMS and MDN into 200 mL OG aqueous solution successively. The pH of reaction solution was adjusted to a desired value by 1 M H₂SO₄ or NaOH before adding other reaction reagents into the solution. At given time intervals, samples (1.0 mL) were withdrawn immediately and then quenched by 1.0 mL phosphate buffered solution (PBS, pH 7.0) and 1.0 mL sodium thiosulfate (0.5 M). The mixtures were filtrated with 0.45 μm membrane before analysis. Unless otherwise stated, the initial conditions were set as [PMS] = 1 mM, [MDN] = 0.25 g/L, [Fe²⁺] = 3.0 mg/L, pH = 6.0, and [OG] = 0.1 mM. Moreover, RhB, ARS, TC and NFX with the same molar concentration as OG were also used as targeted contaminants to investigate the oxidative activity of the MDN/Fe²⁺/PMS system.

Radical quenching tests were conducted by adding a certain concentration of methanol, TBA, phenol, p-BQ, L-histidine or FFA to the reaction solution before the reaction started. High purity nitrogen gas was aerated into the OG solution prior to the reaction started and kept until the experiment finished. Sodium or potassium salts were pre-added into the reaction solution before pH adjustment to study the effects of common anions (Cl⁻, and ⁠). MDN were replaced by pure Mo-based materials (MoS₂, MoO₂, MoO₃ and MoCl₅) with the same dose for comparison, and their reduction reactivities towards Fe³⁺ were also explored in a ferric chloride solution ([Fe³⁺] = 10.0 mg/L). MDN was collected after the reaction finished and then used as co-catalyst in the next cycle to explore its reusability.

Each experiment was performed in duplicate at least, and the results were reported as mean values ± standard deviation in the following text. The significant differences between repeated test results were estimated by one-way ANOVA method on Origin 9.0 software with p values of 0.05.

For excellent degradation of the organic compound, commonly, the dosages of catalyst, the concentrations of Fe²⁺ and PMS, initial solution pH, and inorganic anions are considered as key factors having significant influence over the reaction.

In order to further determine the effect of inorganic anions, the degradation efficiency after adding Cl⁻, and into the reaction system were evaluated. As can be seen from Figure 2(f), the OG removal was inhibited evidently with the addition of 10 mM ⁠, and the removal efficiency decreased to 32.8% within 60 min, but Cl⁻ and had no significant impact. These results could be explained in that was capable to scavenge and strongly, but Cl⁻ and were able to react with and and then produce some weaker radicals such as ⁠, ⁠, Cl₂, and in the presence of Fe²⁺ (Li et al. 2017; Giannakis et al. 2021).

In order to illuminate the potential mechanism of the catalytic degradation of OG in MDN/Fe²⁺/PMS system, the ROS formed in the reaction system were examined by quenching experiments and EPR.

FFA and L-histidine were usually used to verify the presence of singlet oxygen (⁠⁠) (1.2 × 108 M⁻¹ s⁻¹ and 2 × 109 M⁻¹ s⁻¹) in the reaction process (Bokare & Choi 2015; Fu et al. 2019). As shown in Figure 5(b), the degradation rates of OG decreased in a significant way no matter what the concentration of the scavengers was. These results illustrated that was generated in the MDN/Fe²⁺/PMS process. Comparatively, typical 1:1:1 triplet peaks of adducts signals in the EPR spectra were detected in the MDN/PMS and Fe²⁺/PMS processes (Figure 5(e)), which were much weaker than those in the MDN/Fe²⁺/PMS process. There were no adducts signals that appeared by PMS alone, which means that was almost not generated by PMS self-decomposition.

BQ is a specific quencher to examine superoxide radical (⁠⁠) (Furman et al. 2010). As shown in Figure 5(c) and 5(f), the addition of BQ slightly inhibited OG degradation in the MDN/Fe²⁺/PMS process, though the signals of adducts were much stronger than those in the process of PMS alone, MDN/PMS and Fe²⁺/PMS. The results indicated that was not the main ROS responsible for OG degradation in the MDN/Fe²⁺/PMS process.

It should be noted that the signals of ⁠, and adducts in the MDN/Fe²⁺/PMS process was much stronger than the other three cases, demonstrating that much more ⁠, and were generated. Accordingly, it is reasonable to believe that active species (⁠⁠, ⁠, and ⁠) were the main ROS responsible for OG degradation in the MDN/Fe²⁺/PMS process, and their generation process could be related to the synergistic catalysis effect between MDN and Fe²⁺ for PMS activation.

Combined with the above analysis, we could conclude reasonably that PMS was activated by both MDN and Fe²⁺ in radical and non-radical pathways. Firstly, Fe²⁺ activated PMS and generated ⁠, and ⁠. These generated ROS attacked –N = =N– group and aromatic rings, resulting in decomposition of OG into products with lower molecular weight. Simultaneously, generated Fe³⁺ after reaction could be reduced back to Fe²⁺ directly by low valence sulfur element and Mo(IV) on the surface of MDN. Some of the Mo(IV) on the surface could be regenerated by sulfur in MDN, but considerable amounts of Mo element would be released into solution as molybdate.

Due to the merits of being more available and cheaper relative to some synthetic materials, natural mineral-based heterogeneous catalysts hold a great promise in peroxide activation for degradation of organic pollutants (Lai et al. 2021). MDN, though its main component was MoS₂, performed much more poorly than pure MoS₂ in activating PMS for degradation OG (Figure 3(c)). Thus, a large amount of MDN (3.0 g/L) was required to obtain rapid degradation of organic contaminants (Yuan et al. 2021a). Besides, leached molybdenum ions could not be reduced back to low valent Mo by reducible sulfur, resulting in not full utilization of the reducing capacity of MDN.

Azo dye decomposition was greatly enhanced in the present study by adding a low concentration of ferrous ions into MDN/PMS process, with the MDN dosage reduced from 3.0 g/L to 0.25 g/L. The OG degradation rate in MDN/Fe²⁺/PMS process (Figure 1(a)) was also comparable to that in MoS₂/PMS process (Figure 3(c)). In addition, the Mo leaching was suppressed significantly (Figure 1(c)), which had very important environmental benefits. As we know, Mo is toxic and must be controlled before being discharged from industrial processes to natural waters and soils (Huang et al. 2012). The reducing capacity of MDN had been more fully utilized by adding ferrous ions, for the reason that Fe²⁺ could be regenerated through direct reduction of Fe³⁺ with reductive sulfur on its surface (Figure 3). This can also be observed from Figure 1(d), where MDN could be reused for five cycles, much longer than that of two cycles in MDN/PMS process (Yuan et al. 2021a), suggesting that the utilization efficiency of MDN was significantly improved. To our surprise, the main ROS responsible for OG degradation, such as ⁠, and ⁠, were no difference with or without ferrous ions (Figure 5). Overall, the addition of ferrous ions reduced the dosage of MDN and the release of toxic Mo ions extensively. This strategy is conducive to promoting applications of natural minerals activated peroxide process in the field of industrial organic wastewater treatment.

In this study, the synergistic activation of PMS by MDN and Fe²⁺ was evaluated by organic contaminants degradation. PMS activation by MDN was facilitated greatly in the presence of Fe²⁺. MDN mainly acted as the reductant to promote cycling of Fe³⁺/Fe²⁺ redox couple through S and Mo(IV) on its surface. The addition of Fe²⁺ not only reduced the dosage of MDN and the leaching of toxic Mo ions but also improved the utilization efficiency of this mineral catalyst. The initial pH, dosage of MDN, Fe²⁺ and PMS affected degradation rate obviously. Co-existing anions Cl⁻ and had a slight effect, but inhibited OG removal remarkably. The main ROS responsible for OG degradation were identified as ⁠, and ⁠. In conclusion, the present work provides a new method of activating PMS by MDN and Fe²⁺ to degrade organic pollutants in aqueous solution.

This work was supported by the National Natural Science Foundation of China (grant numbers 52170132), Cultivation Project of National Natural Science Foundation of China [grant number 2020PL19] and Foundation of Henan Educational Committee (grant number 21A610006).

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

The authors declare there is no conflict.