Pyrite/H₂O₂/hydroxylamine system for efficient decolorization of rhodamine B

Dye effluent released from textile industries is toxic and carcinogenic, which can cause a severe impact on human health and water ecosystems (Brillas & Martínez-Huitle 2015; Banazadeh et al. 2016; Javaid & Qazi 2019). Even low concentrations (<1.0 mg L⁻¹) of dye can cause adverse effects in water (Javaid & Qazi 2019). Many nations have rigorous regulations on the discharge of textile industry wastewater, which must be properly treated to eliminate the excessive concentrations of chroma and chemical oxygen demand (COD) (Mahalingam et al. 2017). Therefore, there is a strong demand for efficient and advanced treatment technologies. Among the existing wastewater treatment technologies, advanced oxidation processes (AOPs) have attracted the attention of many researchers, because AOPs can convert most organic pollutants into smaller compounds or even CO₂ due to the highly effective reactive oxygen species (ROS) produced during the reaction process (Bagal & Gogate 2014; Zhu et al. 2019; Sun et al. 2021).

Hydroxylamine (NH₂OH, HA) is a common reducing agent (Sun et al. 2020a). It has been reported that HA can enhance the Fe(II)/Fe(III) redox cycle in Fenton-like reactions (Chen et al. 2011; Zou et al. 2013; Hou et al. 2017). In addition, in the pre-experiments, we found that HA could improve the continuous reactivity of pyrite. However, this role of HA has rarely been systematically studied to the best of our knowledge. Moreover, although HA is toxic, it can be rapidly decomposed into non-toxic inorganic substances, such as NO₂⁻, N₂O, and N₂, in Fenton-like processes (Chen et al. 2011; Zou et al. 2013; Li et al. 2019a). Therefore, HA was selected in this work to develop the dyes decolorization ability of pyrite/H₂O₂ system by boosting the Fe(II)/Fe(III) cycle and improving the continuous reactivity of pyrite. To best of our knowledge, there have been few comprehensive studies on the degradation of organic dye pollutants by pyrite/H₂O₂/HA systems.

In this study, the pyrite/H₂O₂/HA system was proposed to decolorize rhodamine B (RhB, an essential example of xanthene dyes (Diao et al. 2017)). The main objectives were to: (i) evaluate the feasibility of pyrite/H₂O₂/HA system for RhB decolorization; (ii) explore the influencing factors including HA concentration, H₂O₂ concentration, pyrite dosage, RhB concentration, and initial pH; (iii) reveal the mechanism of RhB decolorization by the pyrite/H₂O₂/HA system; and (iv) investigate the influence of HA dosing manners, and degradation performance of pyrite/H₂O₂/HA system to other dye pollutants.

Hydrogen peroxide (H₂O₂, 30 wt.%), RhB, orange II (OR), methyl orange (MO), neutral red (NR), hydroxylamine hydrochloride (NH₂OH·HCl, HA·HCl), ferrous sulfate heptahydrate (FeSO₄·7H₂O), tert-butyl alcohol (TBA), sodium hydroxide (NaOH), ethanol (EtOH), hydrochloric acid (HCl), titanic sulfate (Ti(SO₄)₂), and sulfuric acid (H₂SO₄) were acquired from Chengdu Kelong chemical reagent factory (Chengdu, China). 8-Hydroxyquinoline, methylene blue (MB), sodium thiosulfate pentahydrate (Na₂S₂O₃·5H₂O), sodium acetate trihydrate (CH₃COONa·3H₂O), potassium dihydrogen phosphate (KH₂PO₄), 1,10-phenanthroline, and sodium carbonate (Na₂CO₃) were bought from Xilong Science Co., Ltd (Shantou, China). All the reagents were of analytical grade and utilized as obtained without further purification. Deionized water was used in all experiments. Pyrite samples were bought from Wanbao Mining Co., Ltd (Beijing, China). After the sample was ground, it was immersed in 0.1 M HCl to keep its purity of the surface. Then the sample was washed with EtOH and deionized water, and latterly placed in a vacuum drying oven (DZF-6021, Shanghai, China) at 60 °C until it was dry. Finally, it was kept in a closed glass bottle to avoid oxidation.

Batch experiments of RhB decolorization were carried out in a 100 mL flat-bottom beaker containing about 100 mL reaction solution. The beaker was placed on a magnetic stirrer with constantly stirring by a magnetic stir bar at room temperature (25 ± 2 °C). A stock solution of HA (40 mM; prepared daily) or RhB (1.0 g L⁻¹) was stored in a 100 mL volumetric flask and kept in a dark cabinet to prevent photochemical reactions. For a typical test procedure, preset volumes of RhB and HA from the stock solution were poured into the beaker, and next the initial pH was adjusted to a predetermined value with diluted NaOH and H₂SO₄. Then, a specific dose of pyrite and a certain volume of 30 wt.% H₂O₂ were added into the reaction solution to trigger the oxidation reaction. At the preset interval, 1.0 mL reaction suspension was sampled and quenched by 1.0 mL EtOH. Then, the suspension was filtered through a 0.22 μm membrane filter for further analysis. To test the continuous reactivity of pyrite, the pyrite was separated from the solution by centrifugation at 4,000 rpm (TDL-5A, Changzhou, China) after 30 min reaction time, and then washed with EtOH, and lastly dried in the vacuum drying oven at 60 °C for 8 h. The recycled pyrite was reused by adding the same amounts of RhB, HA, and H₂O₂. Unless otherwise specified, [RhB]₀ = 50 mg L⁻¹, [pyrite]₀ = 0.4 g L⁻¹, [HA]₀ = 0.8 mM, [H₂O₂]₀ = 1.6 mM, and initial pH = 4.0. All experiments were conducted in duplicate and the data were averaged. The error bars in the figures represent the standard deviation.

Figure 1(a) shows the XRD pattern of pyrite. The pattern demonstrates that the diffraction peaks are noted at 2θ of 28.5°, 33.0°, 37.1°, 40.8°, 47.4°, 56.3°, 59.0°, 61.7°, 64.4°, 76.6° and 79.0°, corresponding to the (1 1 1), (2 0 0), (2 1 0), (2 1 1), (2 2 0), (3 1 1), (2 2 2), (2 3 0), (3 2 1), (3 3 1) and (4 2 0) characteristic planes of FeS₂ (JCPDS card no. 42-1340), respectively. The results indicated that pyrite has a good crystallized phase and cubic structure. The particle size distribution graph of pyrite is presented in Figure 1(b), and its size was macroscopically mainly distributed in the range 2–50 μm.

Figure 3(d) presents the effect of RhB concentration on RhB decolorization by the pyrite/H₂O₂/HA system. The results display that the decolorization efficiency of RhB drops from 100% to 90.6% with the increase in RhB concentration from 20 to 80 mg L⁻¹. The decrease of decolorization performance is probably attributed to the inadequate generation of radicals for RhB decolorization with constant chemical dosages in the pyrite/H₂O₂/HA system (Li et al. 2019b). Moreover, the number of intermediates produced by the decolorization of RhB will increase with the increase in RhB concentration, which will reduce the chance of RhB decolorization by radicals. Therefore, 50 mg L⁻¹ was selected as the suitable RhB concentration in this research for practical and economic considerations.

According to previous studies, the solution pH frequently has a significant impact on organic pollutants degradation in Fenton-like reactions (He et al. 2020; Zhao et al. 2020; Zhu et al. 2020). Here, experiments were performed to investigate the effect of initial pH on RhB decolorization efficiency by the pyrite/H₂O₂/HA system. As shown in Figure 3(e), the RhB decolorization efficiency was higher than 90% when the initial pH was in the range 3.0–10.0, but it declined significantly when the initial pH was 2.0 (56.8%) or 11.0 (28.4%). To explain the results, the variation of solution pH during the RhB decolorization under different initial pH was examined. The result (Figure 3(f)) shows that the solution pH quickly declined and finally stays in the range 2.5–4.0 (working pH range of Fenton reaction (Luo et al. 2019)) when the initial pH is in the range 3.0–10.0, leading to the fast decolorization of RhB. However, the variation of solution pH was negligible under high acidic (initial pH 2.0) and alkaline (initial pH 11.0) conditions due to the strong buffer capacity at extreme pH conditions (Li et al. 2021). For initial pH 2.0, the generation of Fe²⁺ was delayed by the reactions of pyrite with dissolved O₂ (Equation (1)) and Fe³⁺ (Equation (3)) under these high acidic conditions. Meanwhile, many •OH are consumed by HA (Equation (12)) and Fe²⁺ (Equation (14)). Therefore, the decolorization efficiency of RhB decreased at the initial pH of 2.0. For initial pH 11.0, the low decolorization efficiency of RhB can be explained by the following reasons under this extreme pH condition: (a) the rapid oxidation of the pyrite surface (Todd et al. 2003) and the low solubility of Fe²⁺ led to the low concentration of Fe²⁺ in solution, which inhibited the Fenton reaction. (b) NH₂OH is the primary form of HA as calculated by Equations (9) and (10) in this pH condition, which has a higher reaction rate (k = 9.5 × 10⁹ M⁻¹s⁻¹ (Neta et al. 1988)) with •OH than that (k < 5.0 × 10⁸ M⁻¹s⁻¹) of NH₃OH⁺ (the dominant form of HA as pH < 5.96). Thus, more •OH were quenched by NH₂OH rather than RhB decolorization. In general, high RhB decolorization efficiency can be achieved at a wide-range pH of 3.0–10.0 by the pyrite/H₂O₂/HA system, and 4.0 was selected as the optimal initial pH.

According to previous research, EtOH is a common scavenger for both •OH and SO₄^•− due to the high reaction constant of EtOH with •OH (k = (1.2–2.8) × 10⁹ M⁻¹s⁻¹) and SO₄^•− (k = (1.6–7.7) × 10⁷ M⁻¹s⁻¹) (Anipsitakis & Dionysiou 2004). TBA is frequently used as a quenching agent for •OH with a high reaction rate constant of (3.8–7.6) × 10⁸ M⁻¹s⁻¹ (Anipsitakis & Dionysiou 2004). The concentration of both EtOH and TBA was selected as 800 mM, which corresponds to the molar ratio (500:1) of alcohols to H₂O₂. This technique has been widely applied by Burrows' research team (Hickerson et al. 1998; Stemmler & Burrows 2001). As shown in Figure 4, compared with the decolorization efficiency (99.6%) without scavenger, the decolorization efficiency with EtOH and TBA scavenger decreased to 2.0% and 2.1%, respectively. The results demonstrated that •OH is the dominant radical responsible for RhB decolorization in the pyrite/H₂O₂/HA system.

In order to investigate the role of HA in RhB degradation by the pyrite/H₂O₂/HA system, the concentration changes of Fe_total and Fe²⁺ in solution in pyrite/H₂O₂ and pyrite/H₂O₂/HA systems respectively with 800 mM EtOH (to prevent the reaction between Fe²⁺ and •OH via Equation (14)) but without RhB (to exclude the effect of RhB on the determination of Fe²⁺ and Fe_total (Figure S1a)) were examined. As exhibited in Figure 5(a), the concentration change of Fe_total in solution was almost the same in pyrite/H₂O₂ and pyrite/H₂O₂/HA systems, both increasing from about 8.0 mg L⁻¹ at 0.5 min to 14.7 mg L⁻¹ at 30 min. However, Fe²⁺ concentration in the pyrite/H₂O₂/HA system was about 3–5 times that of the pyrite/H₂O₂ system. The results revealed that HA can boost the Fe(II)/Fe(III) cycle in solution and then enhance RhB decolorization. Furthermore, to further verify the function of HA in the pyrite/H₂O₂/HA system, the concentration variation of H₂O₂ in pyrite/H₂O₂ and pyrite/H₂O₂/HA systems was compared (Figure 5(b)). This showed that H₂O₂ was completely decomposed within 12 min in the pyrite/H₂O₂/HA system. In contrast, the corresponding value was 68% in the pyrite/H₂O₂ system, suggesting that HA can accelerate the decomposition of H₂O₂ and thus generate more •OH radicals to improve the decolorization efficiency of RhB. In addition, considering the toxicity of HA, the concentration change of HA in the pyrite/H₂O₂/HA system was detected. The results (Figure 5(b)) revealed that complete decomposition of HA was achieved in just 8 min, showing the toxicity of HA can be quickly eliminated during the reaction process. Based on these results, the role of HA in the pyrite/H₂O₂/HA system was that it can accelerate the Fe(II)/Fe(III) cycle via the fast reaction with Fe³⁺ (Equation (11)), and thus enhance the decomposition of H₂O₂ to produce more •OH radicals, and finally increase RhB decolorization efficiency.

In order to further investigate the role of HA, the iron and sulfur components on pyrite surface before and after reaction in pyrite/H₂O₂ and pyrite/H₂O₂/HA systems were detected by XPS (Figure 6). Figure 6(a), 6(c) and 6e show the Fe 2p_3/2 spectra of pyrite before and after reaction in pyrite/H₂O₂ and pyrite/H₂O₂/HA systems, respectively. Three peaks at 707.3, 709.1 and 711.1 eV corresponded to Fe(II)-S, Fe(II)-O and Fe(III)-O species, respectively (Cai et al. 2009). The S 2p spectra of pyrite before and after reaction in pyrite/H₂O₂ and pyrite/H₂O₂/HA systems are shown in Figure 6(b), 6(d) and 6f, respectively. Three peaks at 162.6, 163.9 and 168.9 eV were attributed to S₂²⁻ (S(-I)), S_n²⁻ (S(0)) and SO₄²⁻ (S(VI)), respectively (Cai et al. 2009). The proportions of ferrous iron to total iron (Fe(II)/Fe_t) and S(-I) to total sulfur (S(-I)/S_t) on pyrite surface were calculated respectively in light of the fitting peak areas of Fe 2p_3/2 and S 2p core-level spectra (Table S1, supplementary information). For the Fe 2p_3/2 spectrum of pyrite, the proportion of Fe(II)/Fe_t decreased from 79.63% (before reaction) to 49.94% (after reaction) in the pyrite/H₂O₂ system due to the oxidation of pyrite via Equations (1)–(3). Surprisingly, the proportion (79.95%) of Fe(II)/Fe_t after the reaction in the pyrite/H₂O₂/HA system was even slightly higher than that before the reaction because of the reduction of HA. For the S 2p spectrum of pyrite, the proportion of S(-I)/S_t decreased from 52.77% (before reaction) to 36.35% (after reaction) in the pyrite/H₂O₂ system. While the corresponding value in the pyrite/H₂O₂/HA system just decreased to 43.74% (after reaction). The results indicate that HA can greatly promote the Fe(II)/Fe(III) cycle on the pyrite surface and inhibit the consumption of S₂²⁻ sites to avoid the oxidation of pyrite. According to previous studies (Diao et al. 2017; Zhou et al. 2018), the oxidation of pyrite will decrease its reuse reactivity. Therefore, the higher continuous reactivity of pyrite may be achieved in the pyrite/H₂O₂/HA system than that of the pyrite/H₂O₂ system. In order to prove this hypothesis, the reuse reactivity of pyrite in pyrite/H₂O₂ and pyrite/H₂O₂/HA systems was investigated. As shown in Figure 5(c), the RhB decolorization efficiency declined by 15.1% in the pyrite/H₂O₂/HA system after five cycles for 30 min treatment, but the corresponding value was 56.7% in the pyrite/H₂O₂ system. The results suggested that pyrite has higher continuous reactivity in the pyrite/H₂O₂/HA system. Therefore, the other role of HA is that it can improve the continuous reactivity of pyrite by inhibiting its oxidation.

According to the results in Figure 5(a), the concentration of leached iron in the pyrite/H₂O₂/HA system is high. Therefore, the effect of leached iron on RhB decolorization was investigated. Here, 0.04 g pyrite (0.4 g L⁻¹) was put into a RhB solution, the solution was stirred for 30 min, and then it was filtered. The absorbance of the filtrate was measured as A₀, then HA and H₂O₂ were added to the filtrate. Samples were taken at different times, their absorbance was measured, and their decolorization efficiency was calculated via Equation (5). The results are shown in Figure 7. Nearly 93.0% RhB was decolorized by the leached iron/H₂O₂/HA system, which is just slightly lower than that (99.6%) of the pyrite/H₂O₂/HA system. The results suggested that the leached iron has a dominant effect on RhB decolorization, indicating that the pyrite/H₂O₂/HA system is mainly a homogeneous oxidation system.

Based on the above analysis and previous reports (Chen et al. 2015b; Feng et al. 2018; Zhou et al. 2018; Li et al. 2021), the possible reaction mechanism of RhB decolorization by the pyrite/H₂O₂/HA system is clarified by Figure 8. Fe²⁺ leached from the pyrite can react with H₂O₂ to produce •OH radicals for RhB decolorization. This procedure is promoted by HA by accelerating the Fe(II)/Fe(III) cycle via the fast oxidation of HA, and thus stimulating the decomposition of H₂O₂ to generate more •OH radicals. Meanwhile, HA can inhibit the oxidation of pyrite and enhance its continuous reactivity. In addition, HA can directly activate H₂O₂ to generate •OH radicals to degrade RhB.

As presented in Figure 5(b), the H₂O₂ consumption of the pyrite/H₂O₂/HA system was 37% higher than that of the pyrite/H₂O₂ system, while the RhB decolorization efficiency of the pyrite/H₂O₂/HA system was just about 20% higher than that of the pyrite/H₂O₂ system (Figure 2). This phenomenon may be caused by the quenching effect of HA on •OH (Equation (12)). Therefore, to investigate the quenching effect of HA on •OH and reduce it, experiments with different HA dosing manners were performed (Figure 9). A total amount of 0.8 mM HA was either dosed once at a predetermined time after the reaction start or dosed evenly at distinct stages into the reaction solution. Meanwhile, to simulate practical application (high concentration of dye wastewater), the concentration of 100 mg L⁻¹ RhB was selected in the experiments. As shown in Figure 9, the RhB decolorization efficiency of HA dosed once at 4 min (92.3%) or 8 min (86.7%) after the reaction start was higher than that (83.5%) when dosed initially. The decolorization efficiency increases instantly when HA is dosed due to the Fe(II)/Fe(III) redox cycle promoted by the added HA. Surprisingly, more than 97% of RhB was decolorized when HA was dosed evenly at 0, 4 min and 0, 4, 8 min, respectively, which was mainly attributed the addition of HA in batches that weakened its quenching effect on •OH. Therefore, the suitable dosing manner of HA was multiple dosing.

In order to assess the applicability of the pyrite/H₂O₂/HA system, four other dye pollutants, namely OR, MO, NR and MB were treated using it. The results (Figure 10(a)) displayed that complete decolorization was achieved for four dye pollutant and RhB by the pyrite/H₂O₂/HA system only within 12 min, indicating its excellent decolorization ability in dye pollutants treatment. Moreover, the mineralization efficiencies of the four dye pollutants treated by pyrite/H₂O₂/HA system were also examined (Figure 10(b)). This showed that the mineralization efficiencies of OR, MO, NR, MB and RhB were about 61.8%, 58.7%, 63.7%, 60.9% and 52.8%, respectively, after 30 min treatment, suggesting the excellent performance of the pyrite/H₂O₂/HA system for dye pollutant mineralization. These outcomes show that the pyrite/H₂O₂/HA system has low selectivity for dye degradation, indicating that it is a promising technique for dye pollutant treatment.

In this research, the pyrite/H₂O₂/ HA system was proposed for efficient RhB degradation in a wide pH range 3.0–10.0. Its influence factors were investigated and the optimum conditions were obtained as pyrite 0.4 g L⁻¹, H₂O₂ 1.6 mM, initial pH 4.0, HA 0.8 mM and HA added multiple times. The mechanism study showed that •OH was the primary reactive species. HA had two key roles: one was to promote the Fe(II)/Fe(III) cycle via fast self-decomposition, and thus motivate the decomposition of H₂O₂ to produce more •OH radicals, and finally increase the RhB decolorization; the other was to improve the continuous reactivity of pyrite by inhibiting its oxidation. Moreover, H₂O₂ was mainly activated by the leached iron ions in solution. Complete decolorization and above 52% mineralization efficiency of five dyes for 30 min treatment showed that the pyrite/H₂O₂/HA system had low selectivity for dye degradation, and it had good application prospects in organic pollutant treatment. The performance optimization of reducing agents, the toxicity evaluation of degradation intermediates, and the application of the pyrite/H₂O₂/HA system in actual dye wastewater need further study.

None.

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