Heterogeneous activation of H₂O₂/Na₂S₂O₈ with iron ore from water distribution networks for pollutant removal

Perchloroethylene (PCE) is an important organic solvent, which is widely used in the cleaning industry of machinery, electronic parts and clothing, as well as in the production of medicine, textiles, and chemicals (Zhong et al. 2019). Chloramphenicol (CAP) is a bacteriostatic broad-spectrum antibiotic, which is widely used in disease treatment, animal husbandry and aquaculture (Cao et al. 2020). PCE and CAP have been detected in surface water, ground water and even drinking water because of the unreasonable usage and disposition. PCE and CAP can cause various diseases including leukemia, kidney and liver tumors (Zhong et al. 2019), bone marrow depression and aplastic anemia (He et al. 2020) which are serious and widespread problems (Shapiro et al. 2004; Walters et al. 2010).

Various remediation technologies were used to remove PCE and CAP from water. It is well-known that the advanced oxidation processes (AOPs) based on hydroxyl radicals (•OH) and sulfate radicals (⁠⁠) are considered as effective methods to mineralize organic pollutants (Wu et al. 2018). Hydrogen peroxide (H₂O₂, HP), peroxymonosulfate(KHSO₅·0.5KHSO₄·0.5 K₂SO₄, PMS) and persulfate(Na₂S₂O₈, PS) are the common oxidants to produce radicals for degrading pollutants (Luo et al. 2021a). The •OH-AOPs and -AOPs have their respective advantages. The •OH-AOPs have a high reaction rate and are nonselective for organics, -AOPs are stable and selective and has a longer half-life time than •OH. However, in order to achieve the same efficiency, the PS or PMS dosage must be very high which leads to higher salinity in the system (Gu et al. 2019). The dual-oxidant system could reduce oxidant usage and utilize their respective advantages in removal pollutants and have faster degradation rate (Chen et al. 2020; He et al. 2020; Song et al. 2020). The combined application of HP and PS to generate •OH and has been studied to treat various pollutants including levofloxacin (Epold et al. 2015), erythromycin (Li et al. 2017), trichloroethylene (Yan et al. 2013), 2,4-dinitrotoluene (Xu et al. 2019), and even industrial wastewater (Chen et al. 2020).

Iron-containing materials are the most commonly used catalysts to activate HP, PS and PMS because of its cost-effectiveness and eco-friendliness (Rastogi et al. 2009; Romero et al. 2010). Some researchers found that the pipe deposits are iron-rich including α-FeOOH, γ-FeOOH, γ-Fe₂O₃ and Fe₃O₄, etc. (Oliveira et al. 2012; Cui et al. 2016). The pipe deposits could be used as inexpensive catalysts in heterogeneous Fenton-like oxidation, which attracted increasing attention (Oliveira et al. 2012). In our previous study, we explored the adsorption and degradation capacity of simulated pipe deposits (SGR, the mixture of the major constituents of actual pipe deposits (α- and γ-FeOOH)) for 5-bromosalicylic acid(5-BSA), 32.65% and 87.67% of 5-BSA could be removed by adsorption and SGR/H₂O₂ oxidation (Huang et al. 2021). The findings of previous study would make an important contribution to pipe deposits recycling. However, the removal time was too long, and the mechanisms and kinetics were not discussed in detail.

In this study, we analyzed and prepared the main composition of pipe deposits, γ-FeOOH, which is used as a catalyst to activate HP/PS in order to produce radicals for PCE and CAP removal. The hydroxylamine (NH₂OH, HA) was introduced to increase the degradation rate and shorten the degradation time. Furthermore, the underlying mechanism was investigated by radical scavenging test (RST), electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). The method may be expected to be used for in situ treatment of polluted water of the network emergency contamination.

PCE was obtained from Tianjin Bodi Chemicals Co., Ltd FeCl₂·4H₂O and NaOH were purchased from Sinopharm Chemical Reagent Co., Ltd CAP, H₂O₂, Na₂S₂O₈, NH₂OH, para-chlorobenzoic acid (pCBA) and 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) were obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. Methanol(MeOH), tert-butanol(TBA) and isopropanol were from Tianjin Guangfu Fine Chemical Research Institute. All the chemicals were used without any further purification. All solutions were prepared by deionized water.

Lepidocrocite (γ-FeOOH) was synthesized at 25 °C. Firstly, 4 g FeCl₂·4H₂O were added to 100 mL deionized water and continuously stirred, and maintained the solution pH at 6.8 ± 0.1 by adding 1.0 mol L⁻¹ NaOH. Then, air (36 − 72 L h⁻¹) was sent into the solution and maintained pH at 6.8 ± 0.1 until the color of suspension changed from dark green to orange. Finally, the prepared γ-FeOOH was centrifuged and washed with deionized water and dried (Antony et al. 2005).

All batch removal experiments were conducted in a 100 mL capped bottle at 25 °C. A certain number γ-FeOOH were added to 100 mL aqueous solution containing PCE or CAP. HA, HP and PS were added to the abovementioned solution at appropriate concentration to perform degradation experiments. 0.1 mol L⁻¹ HCl and 0.1 mol L⁻¹ NaOH was used to adjust the pH of pollutant solution. The PCE was extracted by n-hexane and the residual concentration was detected by GC-ECD. The CAP residual concentration was detected by a UV-vis spectrophotometer at 277.6 nm. The detected peak did not shift which indicated that the addition of reagents did not influence CAP detection, as shown in Fig. S1. All experiments were conducted in triplicate.

Scanning electron microscope (SEM, ZEISS Sigma 500, Instrument Parameter: EHT = 15.00 kV, Signal A = SE2, WD = 8.4 mm, Mag = 20.00 KX) was used to observe the morphology of γ-FeOOH. X-ray diffraction with 2θ range from 10 to 90° was used to analyzed the crystalline structure of γ-FeOOH(XRD, AXS D8 ADVANCE with CoKα radiation (λ = 1.54060 Å)). X-ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD using Al Ka radiation (1,486.6 eV)) were used to explore the surface valence state of γ-FeOOH before and after degradation. The chemical groups on γ-FeOOH surface were detected by Fourier transformation infrared spectroscopy (FTIR, Perkin Elmer Spectrum One). The main radicals were detected by electron paramagnetic resonance (EPR, Bruker, Germany) with DMPO as the spin-trapping agent. The degradation products were determined by high-resolution LC-MS in negative ionization mode using an Agilent 1260 HPLC coupled with ABSciexQTrap 5500 MS with an electrospray ionization (ESI) source. The MS instrumental parameters were set as follows: source temperature, 500 °C; Gas I, 50 arbitrary units; Gas II, 50 arbitrary units; negative ion spray voltage, 4500 V; curtain gas, 50 arbitrary units; entrance potential (EP), 10 V; declustering potential (DP), 100 V; collision energy (CE), 30 V; collision cell exit potential (CXP), 20 V; PIS scan range, 100–1000 amu.

γ-FeOOH was a rod-type structure by the SEM observation (Figure 1(a)). The XRD patterns (Figure 1(b)) showed that the diffraction peaks at 2θ = 14.16°, 27.16°, 36.14° and 46.89° were attributed to (200), (210), (301) and (501) lattice planes of Lepidocrocite, syn (JCPDS No. 44-1415) (Shao et al. 2022). The stronger intensity and narrower half peak width suggested that γ-FeOOH was a consistent crystal structure. According to the FTIR spectrum (Figure 1(c)), the peak appeared at 1021 cm⁻¹ was the in-plane of Fe-O-H bending vibrations, while the peak appeared at 753 cm⁻¹ was for the out-plane (He et al. 2018). These results verified the successful preparation of γ-FeOOH. Figure 1(d) showed that the N₂ adsorption-desorption isotherms of the prepared γ-FeOOH is type IV with H3 hysteresis loop, demonstrating its microporous/mesoporous structure (Li et al. 2019; Liu et al. 2019). Meanwhile, γ-FeOOH has a rather larger specific surface area (117.567 m²/g), average pore size (23.8672 nm) and pore volume (0.6382 cm³/g). The larger specific surface area could expose more active sites, which was conducive to increasing reaction activity (Lyu et al. 2021).

The rate constant of γ-FeOOH/HA/HP/PS system was 0.0173 min⁻¹, which was higher than γ-FeOOH/HA/HP(0.0103 min⁻¹) and γ-FeOOH/HA/PS (0.0023 min⁻¹). The results indicated that the HP/PS system has the unique advantages in pollutant removal because of synergistic effect.

AOPs are complicated systems, and many factors could influence their performance. In order to better clarify the mechanism of dual-oxidant system, the detailed effect of different experimental conditions on CAP degradation were studied.

The influence of the iron ore dosage on CAP degradation was investigated using the catalyst from 1.4 mmol L⁻¹ to 11.2 mmol L⁻¹, as shown in Figure 3(f). The increase of catalyst dosage provided more active sites for the reaction of HP/PS with antibiotics, resulting in high degradation rate. In contrast to our previous studies about the goethite activating HP/PS system (He et al. 2020), the lepidocrocite system did not have an induction period because the lepidocrocite has a larger specific surface area than goethite, which causes more ≡Fe³⁺ to be reduced and to accumulate more radicals with the catalyst dosage at the same time.

The Figure 4(a) exhibited the residual HP and PS concentration in γ-FeOOH/HA/HP/PS system. After 100 min, the consumption of HP and PS was 75.37% and 42.81%, and the decomposition of PS was two-stage reaction. The Figure 4(b) and 4(c) were the kinetics fitting curves of HP and PS decomposition in γ-FeOOH/HA/HP/PS system, respectively. The detail kinetic parameters were shown in Table 1. HP decomposition was fitted well by the first-order kinetics model and the rate constant was 0.0139 min⁻¹. PS decomposition was a two-phase reaction. In the initial phase (0–60 min), the rate constant was 0.0076 min⁻¹. In the second phase (60–100 min), the rate constant was 0.0024 min⁻¹. At the beginning of the reaction, PS was rapidly decomposed due to the accumulation of a large amount of ≡Fe²⁺ by HA reduction. The HA was consumed which led to the slow conversion of ≡Fe³⁺ to ≡Fe²⁺. Therefore, the PS decomposition entered a slow process.

In order to confirm the primary radicals in CAP degradation by γ-FeOOH/HA/HP/PS system, DMPO was used as a trapping agent to conduct the EPR test. DMPO can react with radicals to generate adducts, such as DMPO-OH and DMPO-SO₄. The strong EPR signal (shown in the Figure 5(a)) was DMPO-OH, simultaneously, the low peaks were most likely to be DMPO-SO₄. PS may be first activated by γ-FeOOH to produce and then react with the adsorbed water to lead to the production of abundant •OH via one-electron transfer process, which was consistent with the H⁺ increase. The similar phenomenon has reported in previous studies (Huang et al. 2014; Duan et al. 2016).

Methanol (MeOH), tert-butanol (TBA) and isopropanol were used as quenchers to perform the quenching tests (Figure 5(b)). The reaction rate constants were = 1.6 − 7.7 × 10⁷ M⁻¹s⁻¹, k_•OH/MeOH = 1.2 − 2.8 × 10⁹ M⁻¹s⁻¹, = 4.0 − 9.1 × 10⁵ M⁻¹s⁻¹ and k_•OH/TBA = 3.8 − 7.6 × 10⁸ M⁻¹s⁻¹, respectively (Wang et al. 2017). The CAP degradation was completely suppressed by MeOH, TBA and Isopropanol, suggesting that •OH was the primary radical in the CAP degradation by γ-FeOOH/HA/HP/PS system (Hou et al. 2017).

The competition kinetic method was used to test the reaction rate constants of CAP with radicals at original pH by using pCBA as the competitor (Cao et al. 2020), as shown in Figure 5(c). According to our previous research, the k_·OH,CAP was 6.10 × 10⁹ M⁻¹s⁻¹ in γ-FeOOH/HA/HP system. The k_·OH,CAP was 4.24 × 10¹⁰ M⁻¹s⁻¹ in the initial phase (0–60 min) and 8.15 × 10⁹ M⁻¹s⁻¹ in the second phase (60–100 min) in γ-FeOOH/HA/HP/PS system, respectively. The rate was higher in the double oxidant system than in the single oxidant system. The result further confirmed the advantages of dual-oxidant system.

The introduction of HA into HP/PS system could lead to a significant increase of the CAP removal efficiency. The degradation efficiency of CAP increased from 3.69% to 81.60% with HA concentration increasing from 0 mmol L⁻¹ to 3 mmol L⁻¹, but the degradation efficiency decreased slightly when the HA concentration increased to 4 mmol L⁻¹, as shown in Figure 3(e). The HA accelerated the reduction of ≡Fe³⁺ to produce ≡Fe²⁺ for activating HP and PS. However, the competitive consumption of radicals with excess HA was the reason for the slight decrease of CAP degradation efficiency (Luo et al. 2021b). The dynamic fitting curves of CAP degradation under different HA dosage was shown in Figure 6(a). The whole reaction process was two-stage reaction kinetics. In the fast reaction stage, plenty of ≡Fe³⁺ was rapidly restored by HA, which caused the fast degradation of CAP. After HA was consumed, the generation of ≡Fe²⁺ was slow, the formation of reactive species was hindered, leading to a follow-up slow oxidation stage (Li et al. 2020).

The XPS spectra of γ-FeOOH before and after the catalytic reaction were favorable to evaluate the changes of Fe (Figure 6(b)) and O (Figure 6(c)) valence state in the HP/PS activation. The peaks at 711.61 eV and 725.21 eV belonged to the binding energy of 2p_3/2 and 2p_1/2 of ≡Fe³⁺ of γ-FeOOH. After degradation, the new peaks at 710.65 eV and 724.25 eV appeared which were ≡Fe²⁺ (Guo et al. 2019a; Wang et al. 2020). And ≡Fe³⁺ and ≡Fe²⁺ were 58.22% and 41.78% of the total iron surface atoms after reaction. This result suggested that the reduction of ≡Fe³⁺ to ≡Fe²⁺ occurred on the surface of γ-FeOOH. In addition, the O 1s peaks could be split into the lattice oxygen (O_lat, Fe-O-Fe and Fe-OH) at 530.03 eV and 531.20 eV, the surface adsorbed oxygen (O_surf) at 532.04 eV and the adsorbed molecular water (O_ads) at 533.50 eV with atomic ratio of 69.84%: 23.78%: 6.38% before the degradation (Baltrusaitis et al. 2007; Kim et al. 2021). After the degradation, the atomic ratio of O_lat, O_surf and O_ads was 45.71%: 37.35%: 16.95%, respectively. The ratio of O_lat reduced, the ratio of O_surf and O_ads increased. The increase in the proportion of O_ads might be attributed to more adsorbed molecular water on the surface of γ-FeOOH. The slight decrease in O_lat was attributed to the surface reduction from ≡Fe³⁺ to ≡Fe²⁺. If the leaching iron concentration was less than 3.92 mg L⁻¹, the process was heterogeneous catalysis (Chou et al. 2001). The leaching concentration of iron ion was 0.843 mg L⁻¹ after 100 min in HP/PS system. All the results confirmed that the degradation process occurs on the γ-FeOOH surface.

The degradation intermediates were detected through LC-MS, and eleven degradation intermediates were observed, and the details are exhibited in Figure 7 and Fig. S2. The oxidation and dehydration of CAP leaded to produce P₁ (Chen et al. 2019). The center C connecting aromatic ring was attacked to produce P₂ and P₃. Simultaneously, the ring was opened to form the intermediate P₄ (Yang et al. 2021). The radical attacked CAP by dechlorinate, dehydration and oxidation that result in the formation of P₅-P₇. The •OH attacked P₅ to generate the hydroxylated product P₈ (Guo et al. 2019b). A further attacking by radicals led to the the formation of P₉-P₁₁.

In brief, the CAP degradation process by γ-FeOOH could be depicted as follows. The HA restored the ≡Fe³⁺ into ≡Fe²⁺. The HP and PS interacted with ≡Fe²⁺ to produce highly oxidative radical species(•OH or ⁠). Then, these radical species attacked CAP and effectively transformed it into intermediates P₁-P₁₁.

In order to resource utilization of pipe deposits from water distribution networks, the primary composition of pipe deposits was employed as catalyst to active HP and PS to produce radicals for degradation pollutant. 92.05% PCE was oxidative degradation after 8 h. 73.31% CAP was removed in 100 min after the HA introduction. Furthermore, the reaction rate was HP/PS(0.0173 min⁻¹) > HP(0.0103 min⁻¹) > PS(0.0023 min⁻¹) and the reaction rate constant of radicals with CAP in HP/PS system was obvious higher. All the results implied that the dual-oxidant system has the unique advantages in pollutant removal. According to the result of XPS and ICP, the CAP degradation by HP/PS system was surface reaction. Eleven degradation intermediates were detected by LC-MS. The pipe deposits from water distribution networks has the capability to remove PCE and CAP in water. Although the effects of water quality and hydraulic conditions were not investigated, this study highlighted the advantage of HP/PS dual-oxidant system and the potential application of pipe deposits in water treatment in case of emerging contamination events.

This work was supported by the National Natural Science Foundation of China (No. 51778177) and Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. 2021TS10).

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.

Dan Zhong: Resources, Funding acquisition; Fu He: Methodology, Investigation, Writing – original draft, Writing – review & editing; Wencheng Ma: Supervision; Yichuan Wu: The PCE degradation test; Jiaju Dong: Project administration.

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