Accelerated Removal of Acid Orange 7 by Natural Iron Ore Activated Peroxymonosulfate System with Hydroxylamine for Promoting Fe(III)/Fe(II) Cycle

Abstract

In this study, peroxymonosulfate (PMS) was activated by cheap and readily available natural iron ore to remove Acid Orange 7 (AO7) in water with the assistance of hydroxylamine (HA). Results show that the presence of HA could accelerate the Fe(II)/Fe(III) cycle on the ore surface, promoting the activation of PMS to generate reactive oxidative species. The effects of ore dosage, PMS dosage, HA dosage and initial pH on the degradation of AO7 were investigated in the HA/Ore/PMS system. Under the optimal conditions, the removal of AO7 could reach 93.1% during 30 min, which was 41.4% higher than the ore/PMS system. The AO7 removal increased with the increase of HA, PMS and ore dosage, but was unaffected by the initial solution pH. Based on radical scavenging experiments and EPR tests, the dominant reactive species in the HA/Ore/PMS system were revealed to be the sulfate radical (SO₄^•−), singlet oxygen (¹O₂), superoxide radical (O₂^•−) and hydroxyl radical (^•OH), which were responsible for the AO7 degradation. Furthermore, the possible reaction mechanism of PMS activation was proposed. This study provides an efficient technique for the removal of azo dye organic contaminant in water, which has great practical significance.

1. Introduction

Azo dyes are typical chemical products commonly used in the printing and dyeing industry [1]. Each year approximately 1.3 × 10⁶ t of dyes are produced across the world, which inevitably results in some serious environmental pollution problems owing to the big discharge of dye wastewater into natural waters. The azo dyes wastewater with deep color, great capacity and high toxicity has already brought severe challenges to the existing water environment and human health. Additionally, this type of wastewater is bio-refractory and hard to be efficiently treated by conventional treatment technologies [2,3]. Therefore, it is necessary to develop an effective technology to efficiently remove the azo dye from the wastewater.

Sulfate-radical (SO₄^•−)-based advanced oxidation processes (AOPs) have been widely studied to remove refractory organic pollutants in wastewater. Of note, the oxidation potential (2.5–3.1 eV) of SO₄^•− is higher than that of the typical hydroxyl radical (^•OH, 1.9–2.7 eV) [4,5,6]. In addition, SO₄^•− has other advantages such as a longer half-life period and higher selectivity for pollutants decomposition, indicating a promising application prospect in water treatment. The solid oxidants peroxymonosulfate (PMS, HSO₅⁻) and peroxydisulfate (PDS, S₂O₈²⁻) are the main sources of SO₄^•−. PMS and PDS are structurally similar to H₂O₂ and both of them have O-O bonds in their chemical structures. Nevertheless, the asymmetric structure of O-O bond in PMS makes it more active and easily decomposes into free radicals via the cleavage of O-O bonds during the PMS activation [7,8,9]. Based on this consideration, PMS activation processes have been widely investigated in wastewater treatment. There are many methods to activate PMS to produce reactive radicals for the removal of organics in water. For instance, homogeneous activation by transition metal ions (Co²⁺, Fe²⁺, Mn²⁺, Cu²⁺, etc.) is a commonly method for PMS activation [10]. However, this process requires an acidic pH environment and probably generates a large amount of sludge, causing the secondary pollution. In order to solve these problems, more and more studies are focusing on the heterogeneous activation process by using solid catalysts as activators [11,12,13,14,15]. Iron-based materials have been widely applied as active substances in Fenton-like reactions owing to their high activity, environmental friendliness and cost-effectiveness.

However, the synthesis process of iron-based compounds by chemicals is complex and costly. In recent decades, natural iron ores have come into view which are cheap and easy to obtain as peroxides activators. It is found that iron ore can activate persulfate to degrade organic contaminants, especially for groundwater and soil remediation, but the activation efficiency also needs to be improved. The main reason was attributed to the low efficiency of the active site Fe(II)/Fe(III) cycle [16]. It is considered that slow reduction of Fe(III) to Fe(II) would result in the accumulation of Fe(III) and the generation of Fe oxide sludge, finally inhibiting the formation of reactive oxygen species to degrade pollutants. To address this issue, a lot of studies proposed to apply some reductants to promote the reduction of Fe(III) [17,18,19,20,21]. The first reducing agent being applied was hydroxylamine (HA) that improved the pollutant removal in Fe(II)/persulfate and Fe(II)/H₂O₂ systems. For instance, Liu et al., employed HA to enhance the oxidation efficiency of sulfamethoxazole in the Fe(II)/PMS system. Compared with Fe(II)/PMS process, the HA/Fe(II)/PMS process showed about four times higher removal efficiency of sulfamethoxazole at pH 3.0 [21].

Further studies also demonstrated that HA displayed remarkable facilitation for the removal of pollutants in iron-based heterogeneous systems. Li reported that the Fe₃O₄/PMS system in the presence of HA could efficiently remove the herbicide atrazine under the near-neutral pH (5.0–6.8) (without buffer) [22], which was mainly due to the highly promoted Fe(III)/Fe(II) cycle by HA. It is noted that HA exists in the form of NH₃OH⁺ under acidic conditions (pH < 5.96), and its reaction rate constant with reactive radicals (e.g., SO₄^•− and ^•OH) is much lower than the rate constant of the reaction between the radicals and most of organic pollutants [23,24], thereby avoiding the senselessly consumption of reactive radicals by HA. In this regard, it is believed that the application of HA in ore activetedPMS system would also enhance the degradation of pollutants. To the best of our knowledge, a few related studies are reported but meaningful in consideration of the practical application by using nature iron ore.

In this work, the HA-enhanced Ore/PMS system was proposed to remove organic contamination in water. The acid orange 7 (AO7), a typical azo dye, was chosen as the target pollutant to study the performance of the HA/Ore/PMS process. The physical and chemical information of ore was characterized by a series of technologies including XRD, SEM, BET, XPS, and so on. The removal efficiencies of AO7 were investigated under the varied catalyst dosage, PMS dosage, HA dosage and initial pH, and the reaction kinetics were analyzed. Based on the radical quenching experiments and electron paramagnetic resonance (EPR) technique, the dominant reactive oxygen species that participated in the oxidation of AO7 have been further discussed and the possible mechanism of PMS activation over the natural ore was proposed. Briefly, This work provides a theoretical basis for the practical application of a natural ore activated PMS system in the remediation of water pollution.

2. Results and Discussion

2.1. Characterization of Ore

X-ray diffraction (XRD) analysis was conducted to investigate the solid phase composition of natural ore. As shown in Figure 1, three strong peaks can be observed at 27°, 35.5° and 45°, which are corresponding to the presence of Fe₃O₄, Fe₃Si and SiO₂, respectively. Some weak peaks were observed, indicating the possible existence of the mixture of iron, silicon and aluminum (FeAl_2.7Si_2.3). These results are in agreement with the component analysis in Section 3.1, which verified that the iron oxides are the main component of ore.

Morphologies of ore were also observed by SEM. The images at different magnifications are shown in Figure 2. It can be seen that the iron ore was irregularly lumpy with a rough surface. Most of the particles were tightly interconnected and the size distribution was not uniform. However, the surface elements seem to be fairly distributed as indicated in Figure 3. It is considered that the rough surface of ore might allow for more active sites for PMS activation and enhance its catalytic performance.

The specific surface area and pore composition of ore are important for the catalytic properties of the catalyst. As shown in Figure 4, the adsorption–desorption isotherm of the ore tends to be type IV (with H3 hysteresis loops) based on the International Union of Pure and Applied Chemistry (IUPAC) test guidelines, suggesting that the sample was a flake or fissure porous material [25]. When the P/P₀ approached 1.0, there was a sharp increase in the adsorption isotherm, presumably due to the presence of large pores in the ore. However, the specific surface area of ore was low and determined to be 0.962 m²·s⁻¹ with the average pore size being 6.805 nm (

Table 1. The specific surface areas results for the iron ore.

Iron Ore	Values
Specific surface area (BET)/(m2 g−1)	0.962
Average pore volume/(cm3 m−1)	0.016
Average pore size/(nm)	6.805

), which might be attributed to the tight aggregation of particles.

2.2. Effect of Different Reaction Systems on AO7 Degradation

To explore the oxidation capacity of the HA/Ore/PMS system, the removal of AO7 was comparatively carried out in different processes including HA/PMS, HA/Ore, Ore/PMS and HA/Ore/PMS (Figure 5a).

As can be seen in Figure 5a, there was almost no AO7 removal after 30 min in both HA/PMS (6.4%) and HA/Ore (1.8%) systems, indicating no adsorption of AO7 and negligible production of reactive oxygen species. The coupling of ore with PMS could remove AO7 by 51.7% under the experimental conditions, which was probably attributed to the generation of reactive radicals in the reaction between ore and PMS. In contrast, the removal of AO7 was greatly improved by introducing 0.05 mM HA in the Ore/PMS system and the removal efficiency of AO7 reached 93.1% within 30 min.It is assumed that the presence of Fe(II) on the ore surface could activate PMS to generate reactive radicals for degrading AO7 molecules. However, Fe(II) would gradually be oxidized to Fe(III) during the oxidation process, resulting in a lot of iron oxide sludge. Thus the degradation rate decreased with the reaction in the later stage. The removal of AO7 followed the pseudo-first-order kinetic (Figure 5b). The removal rate constant in HA/Ore/PMS process was found to be 0.091 min⁻¹, about 4.33 times that in Ore/PMS system (0.021 min⁻¹). As shown in

Table 2. The removal of various pollutants by different catalysts in the HA-enhanced system.

Target Pollutant	Activation System	Removal Rate (%)	k (min−1)	References
Atrazine	HA/Fe3O4/PMS	94	0.152	[22]
Sulfamethoxazole	HA/Fe2O3/PMS	90.8	0.077	[26]
Sulfamethoxazole	HA/CoFe2O4/PMS	100	0.034	[27]
AO7	HA/Ore/PMS	93.1	0.091	This work

, it can be seen that the natural ore is not inferior to these lab-made oxides in terms of degradation ability for organic contaminants, which demonstrates the high catalytic efficiency of the natural ore. The high reaction efficiency in the presence of HA could be ascribed to the significant enhancement of radical generation. The efficient Fe(III)/Fe(II) redox cycle is a vital factor in PMS activation because the rate constant value in Fe(III) reduction is much lower than that in Fe(II) oxidation. The presence of HA could realize the rapid reduction of Fe(III) to Fe(II), enhancing the PMS activation for reactive radicals generation and then improving the oxidation ability of the Ore/PMS system for the AO7 degradation. The total organic carbon (TOC) removal capacity of the HA/Ore/PMS system was also investigated and about 10% TOC removal was obtained after 90 min. The reason for the low TOC removal might be attributed to the small organic acids formed in the decomposition of AO7, which are more stubborn and difficult to be degraded.

2.3. Influencing Factors of HA/Ore/PMS System

2.3.1. Effect of Catalyst Dosage on AO7 Removal

The dosage of catalyst directly relates to the concentration of surface active sites Fe(II)/Fe(III) during the catalytic reaction, thus the effects of catalyst dosage (1 g/L, 2 g/L, 3 g/L) on the AO7 removal need to be investigated.

As shown in Figure 6, when the ore dosage increased from 1 g/L to 3 g/L, the removal efficiency increased with increasing catalyst dosage. The corresponding removal efficiencies of AO7 in 30 min were 81.3%, 93.1% and 96.0%, respectively. It is suggested that a larger amount of catalyst would bring more Fe(II)/Fe(III) to activate PMS. However, a further increase in ore dosage just caused a slight enhancement in the removal efficiency. The reasons might be due to the quenching of partial reactive radicals such as SO₄^•− and ^•OH by excess Fe(II) through Equation (1), thereby reducing the oxidative efficiency of AO7 [28,29].

Fe(II) + SO₄^•−/^•OH → Fe(III) + SO₄²⁻/OH⁻

(1)

2.3.2. Effects of PMS Dosage on AO7 Removal

Figure 7 shows the effect of PMS concentration on the removal of AO7 in the HA/Ore/PMS process. When the PMS dosage increased from 0.25 mM to 1 mM, the removal efficiency of AO7 increased from 46.8% to 90%, respectively. This suggested that the increased concentration of PMS would result in more PMS decomposition into reactive radicals for the AO7 degradation.

Nevertheless, further increase in PMS dosage did not improve the AO7 removal significantly, the removal of AO7 increased slowly to 93% at 2 mM PMS and 93.1% at 3 mM PMS, respectively. As PMS is the source of reactive species, an increase in PMS dosage can produce more reactive species to degrade contaminants. However it should be noted that excess PMS would in turn react with SO₄^•− and ^•OH, finally decreasing the oxidation ability of the HA/Ore/PMS system.

2.3.3. Effects of HA Dosage on AO7 Removal

Figure 8 illustrates the effect of HA dosage on AO7 removal. When the dosage of HA was gradually increased from 0.025 mM to 0.1 mM, the AO7 removal efficiency increased accordingly. As previously reported, the HA as a strong reducing agent can effectively promote the cycle of Fe(II)/Fe(III) in the system and accelerate the generation of free radicals [22,30,31]. It should be worth noting that HA tends to be protonated to form NH₃OH⁺ around pH 3.0, facilitating the Fe(III) reduction into Fe(II). Additionally, under acidic conditions, the reaction rate constants of HA with SO₄^•− and ^•OH are much lower than that of the reaction between radicals and organic pollutants, as well as the reaction between Fe(II) and PMS in an aqueous solution. Consequently, an obvious enhanced removal of AO7 was observed.

2.3.4. Effect of Initial pH on AO7 Removal

It has been reported that the pH value is a crucial factor in the AOPs. The pH value not only affects the formation of active radicals but also affects the existence form of ion and substance species in solution [32]. Hence, it is necessary to evaluate the effect of solution pH on AO7 removal. Figure 9a displays the effect of different initial pH values on AO7 removal in the HA/Ore/HA system. The initial pH of the solution was adjusted to 3, 5, 7, and 9 with a diluted solution of sulfuric acid and sodium hydroxide.

As seen, the removal rates for AO7 were basically the same within 30 min under different initial pH conditions. In order to illustrate this phenomenon, the change of pH in solution during the oxidation process has been monitored, as shown in Figure 9b. It can be observed that all the pH rapidly dropped to around 3 after the addition of PMS. Thus, all the reactions with different initial pH are actually undergone in a similar pH environment, leading to almost the same removal efficiency of AO7.

2.4. Reaction Mechanism

In order to deeply study the mechanism of AO7 degradation, the experiments were conducted by using methanol (towards both SO₄^•− and ^•OH), tert-butanol (towards ^•OH), p-benzoquinone (towards superoxide radical, O₂^•−) and L-histidine (towards singlet oxygen, ¹O₂) as quenching reagents to explore the participated reactive oxygen species in the HA/Ore/PMS system [32,33].

As can be seen in Figure 10, the AO7 removal efficiency significantly dropped to 52.9%, 45.6%, 17.1% and 15.8% within 30 min in the presence of tert-butanol (400 mM), p-benzoquinone (10 mM), methanol (400 mM) and L-histidine (10 mM), respectively. This phenomenon may preliminarily suggest the simultaneous generation of SO₄^•− and ¹O₂ (primary), as well as ^•OH and O₂^•− (secondary), participating in the degradation of AO7 in the HA/Ore/PMS system. Subsequently, EPR tests with DMPO as the spin trapping agent were carried out to further verify the generation of SO₄^•− and ^•OH. As presented in Figure 11, distinct signals corresponding to DMPO-OH and DMPO-SO₄⁻ adducts (derived from ^•OH and SO₄^•−) were observed in the Ore/PMS system, while their intensities were further enhanced with the presence of HA, providing clear evidence for the promoted generation of reactive radicals in HA/Ore/PMS system. Compared with the signal of DMPO-SO₄⁻, the peaks of the DMPO-OH adduct were more obvious, which might be due to the rapid transformation of DMPO-SO₄⁻ into DMPO-OH via nucleophilic substitution.

In the heterogeneous catalytic process, the generation of reactive species mainly occurs on the surface of catalyst. Thus, XPS spectra of the fresh and used ore are compared to understand the change of surface composition and chemical states before and after the reaction. Figure 12a shows Fe, O, Al, Si, Mg, Ca and C elements on the surface of the fresh and used samples, which are consistent with the results from EDS mapping. High-resolution XPS spectra of Fe 2p were displayed in Figure 12b, in which the surface Fe(II) and Fe(III) accounted for 46.07% and 53.93%, respectively, in the fresh catalyst based on the deconvolution of the Fe (2p) envelope. It is considered that once ore was used to activate PMS for the generation of reactive species, a large amount of Fe(II) would be quickly oxidized to Fe(III). However, the surface Fe(III) slightly increased from 53.93% to 54.64% on the used catalyst, probably indicating the enhanced reduction of Fe(III) by HA.

As shown in Figure 12c, three characteristic peaks located at 530.3 eV, 531.8 eV and 532.8 eV were attributed to the lattice oxygen (Fe-O), surface hydroxyl group (−OH) and the O in adsorbed H₂O, respectively. For the fresh ore catalyst, a large number of the surface −OH was found on the surface, which may be beneficial for the catalytic oxidation process. After the oxidation of AO7, the percentage of the −OH component reduced greatly from 75.51% to 66.12%, probably because the surface −OH groups were also active sites in the activation of PMS.

Based on the above results and discussions, a possible reaction mechanism in the HA-enhanced Ore/PMS system was proposed in Figure 13. Remarkably, the ore surface readily adsorbs H₂O molecules that tend to be dissociated into hydroxyl groups (−OH), which was consistent with the XPS results (Figure 12). For the PMS activation by iron ore, the H₂O molecule was first physically adsorbed on the active sites of surface Fe(II) to form ≡Fe(II)−OH [22,32]. Then, ≡Fe(II)−OH reacts with PMS to form SO₄^•− and ^•OH (Equations (2) and (3)). When the HA existed as NH₃OH⁺ species in acidic conditions, it would react with the accumulated ≡Fe(III)-OH, promoting the reduction of ≡Fe(III)-OH to ≡Fe(II)-OH (Equations (4) and (5)), which in turn leads to an increase in PMS activation via Equation (2) to yield a large number of reactive oxidative species like SO₄^•−, ¹O₂, O₂^•− and ^•OH (Equations (6)–(10)) [22,31,32,33,34,35]. Combined with the quenching experiments above, it is assumed that the SO₄^•− and ¹O₂ play a major role in the removal of AO7 in the HA/Ore/PMS system.

≡Fe(II)-OH + HSO₅⁻ → ≡Fe(III)-OH + SO₄^•− + OH⁻

(2)

SO₄^•− + H₂O → SO₄²⁻ + ^•OH + H⁺

(3)

≡Fe(III)-OH + NH₃OH⁺ → ≡Fe(II)-OH + 1/2N₂ + 2H⁺ + H₂O

(4)

2≡Fe(III)-OH + NH₃OH⁺ → 2≡Fe(II)-OH + 1/2N₂O + 3H⁺ + 1/2H₂O

(5)

≡Fe(III)-OH + HSO₅⁻ → SO₅^•− + H⁺ + ≡Fe(II)-OH

(6)

≡Fe(II)-OH + O₂ → ≡Fe(III)-OH + O₂^•−

(7)

2SO₅^•− → 2SO₄²⁻ + ¹O₂

(8)

2O₂^•− + 2H₂O → H₂O₂ + ¹O₂ + 2OH⁻

(9)

O₂^•− + ^•OH → ¹O₂ + OH⁻

(10)

2.5. Recyclability of Catalyst

The recycling tests were conducted to investigate the stability of iron ore in the oxidation process. The catalyst was easily separated, washed and dried at 100 °C for 12 h and finally maintained at room temperature for further use. As shown in Figure 14a, the removal efficiency was above 90.0% after 30 min of reaction in the 1st run. Though the AO7 removal efficiency decreased slightly after the first use, the removal efficiency was still relatively high (>65.0%) in the 2nd and 3rd runs. The decrease in efficiency might be due to the leaching of Fe ions, the adsorbed AO7 degradation intermediates on the catalyst surface and the conglomeration of the catalyst [31]. The leached Fe ions concentration in the solution has been monitored over multiple cycles (Figure 14b). Results indicated that the leaching Fe ions were slightly increased but remained below 0.2 mg/L in three cycles of experiments. All the values were lower than the legal limit (2.7 mg/L) imposed by the directives of the US Environmental Protection Agency, as well as the 10.0 mg/L of discharge limit (GB 8978-2002) regulated by the Chinese Environmental Protection Agency. Figure 14c shows the XRD of the used catalyst. It can be seen that the diffraction peaks at 27° and 45° (corresponding to SiO₂ and Fe₃O₄) of used ore were much lower, indicating that Fe and Si may be released into the solution during the experiments, and the results are consistent with the iron leaching experiment.

3. Materials and Methods

3.1. Materials

The iron ore used in this experiment came from an iron mill in Huangshi City, Hubei Province. They were crushed into small particles by a vertical planetary ball mill (XQM-2, China), afterward poured into a mortar, ground for 10 min, and then passed through a 100 mesh sieve. Ore smaller than 100 mesh was taken for the experiment. The analysis results of the iron phase are shown in

Table 3. The iron phase analysis results of a comprehensive sample of raw ore.

Phase Name	Fe3O4	Fe2(CO3)3	Fe2O3	FeS	Fe2(SiO3)3	Full Iron
Content (%)	24.47	0.87	3.15	1.74	0.35	30.58
Distribution rate (%)	80.02	2.85	10.29	5.70	1.14	100.00

. The macroscale images of ore before and after pretreatment are shown in Figure 15.

Acid Orange 7 (AO7, AR) and potassium peroxymonosulfate (PMS, the active ingredient, KHSO₅, content is 42–46%) were obtained from Macklin. Hydroxylamine hydrochloride (NH₃OHCl, AR), methanol (CH₄O, GR), tert-Butanol (C₄H₁₀O, AR), p-benzoquinone (C₆H₄O₂, CP), L-Histidine (C₆H₉N₃O₂), 1,10-phenanthroline (C₁₂H₈N₂, AR), sodium hydroxide (NaOH, AR) and sulfuric acid (H₂SO₄, AR) were purchased from Sinopharm Group. All of these chemicals were used as received without further purification and the solutions were prepared using deionized water.

3.2. Experimental Procedures

The AO7 degradation experiments were performed in 250 mL beakers at a temperature of 26 ± 0.5 °C with a constant stirring rate. Firstly, a stock solution of 200 mL AO7 (10 mg/L) was placed in a 250 mL glass beaker. Unless otherwise specified, the initial pH of the dye solution was adjusted by using 0.1 M sulfuric acid and 0.1 M sodium hydroxide. Subsequently, a certain amount of PMS and HA were added into the AO7 solution in turn and the required ore iron ore catalyst was added quickly to start timing. At given time intervals, 3.0 mL aliquots were filtered through the 0.45 μm membrane and immediately analyzed. All the experiments were carried out in duplicate.

3.3. Analytical Methods

The AO7 removal efficiency was calculated by measuring the absorbance at 485 nm (the maximum absorption wavelength of AO7) using a UV-9100 spectrophotometer. X-ray powder diffraction (XRD) on MiniFlex 600 (Rigaku, Japan) instrument with Cu Kα radiation. The morphology and composition of the samples were observed on a field emission scanning electron microscope (FESEM-EDS, SIGMA 500). N₂ sorption isotherms were acquired from a physical adsorption apparatus (Quantachrome, (Boynton Beach, FL, USA)). The surface composition of the solid was characterized by X-ray photoelectron microscopy (XPS, Thermo Scientific 250Xi (Waltham, MA, USA)) under ultra-high vacuum (UHV) condition with Al-Kα X-ray. The total iron content analysis was carried out by 1,10-phenanthroline spectrophotometry, and the absorbance of solution containing iron ions was measured at 510 nm. EPR spectroscopy was used to identify the reactive species generated in the HA/Ore/PMS system with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a radical-trapping agent. The concentration of TOC was measured by the vario TOC select (Elementar, Hanau, Germany).

4. Conclusions

In this study, HA was introduced as a reducing agent to accelerate Fe(III)/Fe(II) recycling on the iron ore surface, enhancing the iron ore activated PMS process for the degradation of AO7. In the HA/Ore/PMS system, the AO7 removal was improved with the increase of iron ore, PMS and HA dosages, but it needs to avoid the meaningless scavenging and wasting by the excessive dose. Under the optimal conditions (Ore = 2 g/L, PMS = 2 mM, HA = 0.05 mM), the removal efficiency of AO7 (10 mg/L) could reach 93.1% within 30 min, which demonstrates the high efficiency of the natural ore in PMS activation for organics degradation. Furthermore, it is verified that the reactive species including SO₄^•−, ¹O₂, O₂^•− and ^•OH participated in the oxidation process, in which SO₄^•− and ¹O₂ play a major role in AO7 removal. In conclusion, the HA/Ore/PMS system is an efficient method for the remediation of azo dye-containing wastewater.